From 40f8bdcfb8c7687d8e3ead17bb3825ad6a664e85 Mon Sep 17 00:00:00 2001
From: Anton-Latukha <anton.latukha@gmail.com>
Date: Thu, 18 Feb 2021 18:13:41 +0200
Subject: [PATCH 1/4] Improvements to make it easier to implement runners other
 than StandardT

* Refactor MonadThunk
* Refactor Nix.Thunk.Basic
* Add Nix.Thunk.Separate
* Add Nix.Fresh.Stable
* Add StableId
* Remove Scopes from Context and replace with ScopeT
* Remove the `t` parameter formerly representing thunks. It can now be inferred from `m`

TODO:
  * [ ] Provide implementations for citations class
  * [ ] Restore Nix.Lint
  * [ ] Fix 3 broken tests
---
 hnix.cabal                    |   7 +-
 main/Main.hs                  |  27 +-
 main/Repl.hs                  |  90 ++---
 src/Nix.hs                    |  34 +-
 src/Nix/Builtins.hs           | 620 +++++++++++++++++-----------------
 src/Nix/Cited.hs              |   8 +-
 src/Nix/Cited/Basic.hs        |  36 +-
 src/Nix/Context.hs            |  22 +-
 src/Nix/Convert.hs            | 144 ++++----
 src/Nix/Effects.hs            |  29 +-
 src/Nix/Effects/Basic.hs      |  51 +--
 src/Nix/Effects/Derivation.hs |  31 +-
 src/Nix/Eval.hs               |  73 ++--
 src/Nix/Exec.hs               | 171 +++++-----
 src/Nix/Fresh.hs              |  22 +-
 src/Nix/Fresh/Basic.hs        |  51 +--
 src/Nix/Fresh/Stable.hs       | 102 ++++++
 src/Nix/Json.hs               |   5 +-
 src/Nix/Lint.hs               |   9 +-
 src/Nix/Normal.hs             |  84 ++---
 src/Nix/Pretty.hs             |  39 +--
 src/Nix/Reduce.hs             |  14 +-
 src/Nix/Render.hs             |   8 +-
 src/Nix/Render/Frame.hs       |  16 +-
 src/Nix/Scope.hs              |  62 +++-
 src/Nix/Scope/Basic.hs        |  92 +++++
 src/Nix/Standard.hs           | 197 +++++------
 src/Nix/String/Coerce.hs      |   8 +-
 src/Nix/Thunk.hs              |  99 +++---
 src/Nix/Thunk/Basic.hs        | 102 ++++--
 src/Nix/Thunk/Separate.hs     | 162 +++++++++
 src/Nix/Thunk/StableId.hs     |  81 +++++
 src/Nix/Type/Infer.hs         |  13 +-
 src/Nix/Utils/Fix1.hs         |   4 +-
 src/Nix/Value.hs              | 198 +++++------
 src/Nix/Value/Equal.hs        |  16 +-
 src/Nix/XML.hs                |   4 +-
 tests/NixLanguageTests.hs     |   4 +-
 tests/TestCommon.hs           |  15 +-
 39 files changed, 1624 insertions(+), 1126 deletions(-)
 create mode 100644 src/Nix/Fresh/Stable.hs
 create mode 100644 src/Nix/Scope/Basic.hs
 create mode 100644 src/Nix/Thunk/Separate.hs
 create mode 100644 src/Nix/Thunk/StableId.hs

diff --git a/hnix.cabal b/hnix.cabal
index 3a30b7a87..847bb5444 100644
--- a/hnix.cabal
+++ b/hnix.cabal
@@ -362,8 +362,9 @@ library
       Nix.Frames
       Nix.Fresh
       Nix.Fresh.Basic
+      Nix.Fresh.Stable
       Nix.Json
-      Nix.Lint
+      -- Nix.Lint
       Nix.Normal
       Nix.Options
       Nix.Parser
@@ -372,12 +373,14 @@ library
       Nix.Render
       Nix.Render.Frame
       Nix.Scope
+      Nix.Scope.Basic
       Nix.Standard
       Nix.String
       Nix.String.Coerce
       Nix.TH
       Nix.Thunk
       Nix.Thunk.Basic
+      Nix.Thunk.StableId
       Nix.Type.Assumption
       Nix.Type.Env
       Nix.Type.Infer
@@ -406,11 +409,13 @@ library
     , deepseq >= 1.4.3 && <1.5
     , deriving-compat >= 0.3 && < 0.6
     , directory >= 1.3.1 && < 1.4
+    , exception-transformers >= 0.4 && <0.5
     , exceptions >= 0.10.0 && < 0.11
     , filepath >= 1.4.2 && < 1.5
     , free >= 5.1 && < 5.2
     , gitrev >= 1.1.0 && < 1.4
     , hashable >= 1.2.5 && < 1.4
+    , ghc-prim >= 0.5 && <0.7
     , hashing >= 0.1.0 && < 0.2
     , hnix-store-core >= 0.4.0 && < 0.5
     , hnix-store-remote >= 0.4.0 && < 0.5
diff --git a/main/Main.hs b/main/Main.hs
index c7c29bb87..5b0cd209a 100644
--- a/main/Main.hs
+++ b/main/Main.hs
@@ -26,7 +26,7 @@ import qualified Data.Text.IO                  as Text
 import           Nix
 import           Nix.Convert
 import qualified Nix.Eval                      as Eval
-import           Nix.Fresh.Basic
+import           Nix.Fresh.Stable
 import           Nix.Json
 import           Nix.Options.Parser
 import           Nix.Standard
@@ -34,6 +34,7 @@ import           Nix.Thunk.Basic
 import qualified Nix.Type.Env                  as Env
 import qualified Nix.Type.Infer                as HM
 import           Nix.Utils
+import           Nix.Utils.Fix1
 import           Nix.Var
 import           Nix.Value.Monad
 import           Options.Applicative     hiding ( ParserResult(..) )
@@ -94,8 +95,9 @@ main = do
         NixException frames ->
           errorWithoutStackTrace
             .   show
-            =<< renderFrames @(StdValue (StandardT (StdIdT IO)))
-                  @(StdThunk (StandardT (StdIdT IO)))
+            =<< renderFrames
+                  @(StdValue (StandardT IO))
+                  @(StdThunk (StandardT IO) IO)
                   frames
 
       when (repl opts) $
@@ -138,7 +140,7 @@ main = do
    where
     printer
       | finder opts
-      = fromValue @(AttrSet (StdValue (StandardT (StdIdT IO)))) >=> findAttrs
+      = fromValue @(AttrSet (StdValue (StandardT IO))) >=> findAttrs
       | xml opts
       = liftIO
         .   putStrLn
@@ -159,17 +161,17 @@ main = do
       = liftIO . print . prettyNValue <=< removeEffects
      where
       findAttrs
-        :: AttrSet (StdValue (StandardT (StdIdT IO)))
-        -> StandardT (StdIdT IO) ()
+        :: AttrSet (StdValue (StandardT IO))
+        -> StandardT IO ()
       findAttrs = go ""
        where
         go prefix s = do
           xs <- forM (sortOn fst (M.toList s)) $ \(k, nv) -> case nv of
             Free v -> pure (k, pure (Free v))
-            Pure (StdThunk (extract -> Thunk _ _ ref)) -> do
+            Pure (StdThunk (Thunk _ _ ref)) -> do
               let path         = prefix <> Text.unpack k
                   (_, descend) = filterEntry path k
-              val <- readVar @(StandardT (StdIdT IO)) ref
+              val <- readVar @(StandardT IO) ref
               case val of
                 Computed _ -> pure (k, Nothing)
                 _ | descend   -> (k, ) <$> forceEntry path nv
@@ -211,8 +213,9 @@ main = do
                 .   (k <>)
                 .   (": " <>)
                 .   show
-                =<< renderFrames @(StdValue (StandardT (StdIdT IO)))
-                      @(StdThunk (StandardT (StdIdT IO)))
+                =<< renderFrames
+                      @(StdValue (StandardT IO))
+                      @(StdThunk (StandardT IO) IO)
                       frames
               pure Nothing
 
@@ -224,8 +227,8 @@ main = do
   handleReduced
     :: (MonadThrow m, MonadIO m)
     => FilePath
-    -> (NExprLoc, Either SomeException (NValue t f m))
-    -> m (NValue t f m)
+    -> (NExprLoc, Either SomeException (NValue f m))
+    -> m (NValue f m)
   handleReduced path (expr', eres) = do
     liftIO $ do
       putStrLn $ "Wrote winnowed expression tree to " <> path
diff --git a/main/Repl.hs b/main/Repl.hs
index 7143f82e1..ad184d077 100644
--- a/main/Repl.hs
+++ b/main/Repl.hs
@@ -10,12 +10,15 @@
 {-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE GADTs #-}
 {-# LANGUAGE MultiWayIf #-}
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE TupleSections #-}
 {-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE UndecidableInstances #-}
 
 module Repl
   ( main
@@ -29,6 +32,8 @@ import           Nix.Scope
 import           Nix.Utils
 import           Nix.Value.Monad                (demand)
 
+import           Control.Comonad
+import           Data.Functor.Classes
 import qualified Data.List
 import qualified Data.Maybe
 import qualified Data.HashMap.Lazy
@@ -64,13 +69,13 @@ import qualified System.Exit
 import qualified System.IO.Error
 
 -- | Repl entry point
-main :: (MonadNix e t f m, MonadIO m, MonadMask m) =>  m ()
+main :: (MonadNix e f m, MonadIO m, MonadMask m) =>  m ()
 main = main' Nothing
 
 -- | Principled version allowing to pass initial value for context.
 --
 -- Passed value is stored in context with "input" key.
-main' :: (MonadNix e t f m, MonadIO m, MonadMask m) => Maybe (NValue t f m) -> m ()
+main' :: (MonadNix e f m, MonadIO m, MonadMask m) => Maybe (NValue f m) -> m ()
 main' iniVal = initState iniVal >>= \s -> flip evalStateT s
     $ System.Console.Repline.evalRepl
         banner
@@ -128,11 +133,14 @@ main' iniVal = initState iniVal >>= \s -> flip evalStateT s
 -- * Types
 ---------------------------------------------------------------------------------
 
-data IState t f m = IState
+data IState f m = IState
   { replIt  :: Maybe NExprLoc          -- ^ Last expression entered
-  , replCtx :: AttrSet (NValue t f m)  -- ^ Value environment
+  , replCtx :: AttrSet (NValue f m)    -- ^ Value environment
   , replCfg :: ReplConfig              -- ^ REPL configuration
-  } deriving (Eq, Show)
+  }
+
+deriving instance (Eq1 f, Eq1 m, Eq (Thunk m)) => Eq (IState f m)
+deriving instance (Comonad f, Show (Thunk m)) => Show (IState f m)
 
 data ReplConfig = ReplConfig
   { cfgDebug  :: Bool
@@ -148,7 +156,7 @@ defReplConfig = ReplConfig
   }
 
 -- | Create initial IState for REPL
-initState :: MonadNix e t f m => Maybe (NValue t f m) -> m (IState t f m)
+initState :: MonadNix e f m => Maybe (NValue f m) -> m (IState f m)
 initState mIni = do
 
   builtins <- evalText "builtins"
@@ -164,23 +172,23 @@ initState mIni = do
       , cfgValues = values opts
       }
   where
-    evalText :: (MonadNix e t f m) => Text -> m (NValue t f m)
+    evalText :: (MonadNix e f m) => Text -> m (NValue f m)
     evalText expr = case parseNixTextLoc expr of
       Failure e -> error $ "Impossible happened: Unable to parse expression - '" <> Data.Text.unpack expr <> "' error was " <> show e
       Success e -> do evalExprLoc e
 
-type Repl e t f m = HaskelineT (StateT (IState t f m) m)
+type Repl e f m = HaskelineT (StateT (IState f m) m)
 
 ---------------------------------------------------------------------------------
 -- * Execution
 ---------------------------------------------------------------------------------
 
 exec
-  :: forall e t f m
-   . (MonadNix e t f m, MonadIO m)
+  :: forall e f m
+   . (MonadNix e f m, MonadIO m)
   => Bool
   -> Text
-  -> Repl e t f m (Maybe (NValue t f m))
+  -> Repl e f m (Maybe (NValue f m))
 exec update source = do
   -- Get the current interpreter state
   st <- get
@@ -206,7 +214,7 @@ exec update source = do
 
       case mVal of
         Left (NixException frames) -> do
-          lift $ lift $ liftIO . print =<< renderFrames @(NValue t f m) @t frames
+          lift $ lift $ liftIO . print =<< renderFrames @(NValue f m) frames
           pure Nothing
         Right val -> do
           -- Update the interpreter state
@@ -237,18 +245,18 @@ exec update source = do
     toAttrSet i = "{" <> i <> (if Data.Text.isSuffixOf ";" i then mempty else ";") <> "}"
 
 cmd
-  :: (MonadNix e t f m, MonadIO m)
+  :: (MonadNix e f m, MonadIO m)
   => String
-  -> Repl e t f m ()
+  -> Repl e f m ()
 cmd source = do
   mVal <- exec True (Data.Text.pack source)
   case mVal of
     Nothing -> pure ()
     Just val -> printValue val
 
-printValue :: (MonadNix e t f m, MonadIO m)
-           => NValue t f m
-           -> Repl e t f m ()
+printValue :: (MonadNix e f m, MonadIO m)
+           => NValue f m
+           -> Repl e f m ()
 printValue val = do
   cfg <- replCfg <$> get
   lift $ lift $ do
@@ -262,9 +270,9 @@ printValue val = do
 ---------------------------------------------------------------------------------
 
 -- :browse command
-browse :: (MonadNix e t f m, MonadIO m)
+browse :: (MonadNix e f m, MonadIO m)
        => String
-       -> Repl e t f m ()
+       -> Repl e f m ()
 browse _ = do
   st <- get
   forM_ (Data.HashMap.Lazy.toList $ replCtx st) $ \(k, v) -> do
@@ -273,9 +281,9 @@ browse _ = do
 
 -- :load command
 load
-  :: (MonadNix e t f m, MonadIO m)
+  :: (MonadNix e f m, MonadIO m)
   => String
-  -> Repl e t f m ()
+  -> Repl e f m ()
 load args = do
   contents <- liftIO
     $ Data.Text.IO.readFile
@@ -286,9 +294,9 @@ load args = do
 
 -- :type command
 typeof
-  :: (MonadNix e t f m, MonadIO m)
+  :: (MonadNix e f m, MonadIO m)
   => String
-  -> Repl e t f m ()
+  -> Repl e f m ()
 typeof args = do
   st <- get
   mVal <- case Data.HashMap.Lazy.lookup line (replCtx st) of
@@ -303,11 +311,11 @@ typeof args = do
   where line = Data.Text.pack args
 
 -- :quit command
-quit :: (MonadNix e t f m, MonadIO m) => a -> Repl e t f m ()
+quit :: (MonadNix e f m, MonadIO m) => a -> Repl e f m ()
 quit _ = liftIO System.Exit.exitSuccess
 
 -- :set command
-setConfig :: (MonadNix e t f m, MonadIO m) => String -> Repl e t f m ()
+setConfig :: (MonadNix e f m, MonadIO m) => String -> Repl e f m ()
 setConfig args = case words args of
   []       -> liftIO $ putStrLn "No option to set specified"
   (x:_xs)  ->
@@ -326,8 +334,8 @@ defaultMatcher =
   ]
 
 completion
-  :: (MonadNix e t f m, MonadIO m)
-  => CompleterStyle (StateT (IState t f m) m)
+  :: (MonadNix e f m, MonadIO m)
+  => CompleterStyle (StateT (IState f m) m)
 completion = System.Console.Repline.Prefix
   (completeWordWithPrev (pure '\\') separators completeFunc)
   defaultMatcher
@@ -340,15 +348,15 @@ completion = System.Console.Repline.Prefix
 -- Heavily inspired by Dhall Repl, with `algebraicComplete`
 -- adjusted to monadic variant able to `demand` thunks.
 completeFunc
-  :: forall e t f m . (MonadNix e t f m, MonadIO m)
+  :: forall e f m . (MonadNix e f m, MonadIO m)
   => String
   -> String
-  -> (StateT (IState t f m) m) [Completion]
+  -> (StateT (IState f m) m) [Completion]
 completeFunc reversedPrev word
   -- Commands
   | reversedPrev == ":"
   = pure . listCompletion
-      $ fmap helpOptionName (helpOptions :: HelpOptions e t f m)
+      $ fmap helpOptionName (helpOptions :: HelpOptions e f m)
 
   -- Files
   | any (`Data.List.isPrefixOf` word) [ "/", "./", "../", "~/" ]
@@ -383,9 +391,9 @@ completeFunc reversedPrev word
 
     notFinished x = x { isFinished = False }
 
-    algebraicComplete :: (MonadNix e t f m)
+    algebraicComplete :: (MonadNix e f m)
                       => [Text]
-                      -> NValue t f m
+                      -> NValue f m
                       -> m [Text]
     algebraicComplete subFields val =
       let keys = fmap ("." <>) . Data.HashMap.Lazy.keys
@@ -407,16 +415,16 @@ completeFunc reversedPrev word
 
 -- HelpOption inspired by Dhall Repl
 -- with `Doc` instead of String for syntax and doc
-data HelpOption e t f m = HelpOption
+data HelpOption e f m = HelpOption
   { helpOptionName     :: String
   , helpOptionSyntax   :: Doc ()
   , helpOptionDoc      :: Doc ()
-  , helpOptionFunction :: Cmd (Repl e t f m)
+  , helpOptionFunction :: Cmd (Repl e f m)
   }
 
-type HelpOptions e t f m = [HelpOption e t f m]
+type HelpOptions e f m = [HelpOption e f m]
 
-helpOptions :: (MonadNix e t f m, MonadIO m) => HelpOptions e t f m
+helpOptions :: (MonadNix e f m, MonadIO m) => HelpOptions e f m
 helpOptions =
   [ HelpOption
       "help"
@@ -513,10 +521,10 @@ renderSetOptions so =
          <>  Prettyprinter.line
          <>  Prettyprinter.indent 4 (helpSetOptionDoc h)
 
-help :: (MonadNix e t f m, MonadIO m)
-     => HelpOptions e t f m
+help :: (MonadNix e f m, MonadIO m)
+     => HelpOptions e f m
      -> String
-     -> Repl e t f m ()
+     -> Repl e f m ()
 help hs _ = do
   liftIO $ putStrLn "Available commands:\n"
   forM_ hs $ \h ->
@@ -532,6 +540,6 @@ help hs _ = do
        <>  Prettyprinter.indent 4 (helpOptionDoc h)
 
 options
-  :: (MonadNix e t f m, MonadIO m)
-  => System.Console.Repline.Options (Repl e t f m)
+  :: (MonadNix e f m, MonadIO m)
+  => System.Console.Repline.Options (Repl e f m)
 options = (\h -> (helpOptionName h, helpOptionFunction h)) <$> helpOptions
diff --git a/src/Nix.hs b/src/Nix.hs
index f443a3342..7186a898e 100644
--- a/src/Nix.hs
+++ b/src/Nix.hs
@@ -2,6 +2,7 @@
 {-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE ViewPatterns #-}
 
 module Nix
@@ -58,7 +59,7 @@ import           Nix.XML
 --   type. It sets up the common Nix environment and applies the
 --   transformations, allowing them to be easily composed.
 nixEval
-  :: (MonadNix e t f m, Has e Options, Functor g)
+  :: (MonadNix e f m, Has e Options, Functor g)
   => Maybe FilePath
   -> Transform g (m a)
   -> Alg g (m a)
@@ -68,19 +69,20 @@ nixEval mpath xform alg = withNixContext mpath . adi alg xform
 
 -- | Evaluate a nix expression in the default context
 nixEvalExpr
-  :: (MonadNix e t f m, Has e Options)
+  :: forall e f m.
+     (MonadNix e f m, Has e Options)
   => Maybe FilePath
   -> NExpr
-  -> m (NValue t f m)
+  -> m (NValue f m)
 nixEvalExpr mpath = nixEval mpath id Eval.eval
 
 -- | Evaluate a nix expression in the default context
 nixEvalExprLoc
-  :: forall e t f m
-   . (MonadNix e t f m, Has e Options)
+  :: forall e f m
+   . (MonadNix e f m, Has e Options)
   => Maybe FilePath
   -> NExprLoc
-  -> m (NValue t f m)
+  -> m (NValue f m)
 nixEvalExprLoc mpath = nixEval
   mpath
   (Eval.addStackFrames . Eval.addSourcePositions)
@@ -92,17 +94,17 @@ nixEvalExprLoc mpath = nixEval
 --   'MonadNix'). All this function does is provide the right type class
 --   context.
 nixTracingEvalExprLoc
-  :: (MonadNix e t f m, Has e Options, MonadIO m, Alternative m)
+  :: (MonadNix e f m, Has e Options, MonadIO m, Alternative m)
   => Maybe FilePath
   -> NExprLoc
-  -> m (NValue t f m)
+  -> m (NValue f m)
 nixTracingEvalExprLoc mpath = withNixContext mpath . evalExprLoc
 
 evaluateExpression
-  :: (MonadNix e t f m, Has e Options)
+  :: (MonadNix e f m, Has e Options)
   => Maybe FilePath
-  -> (Maybe FilePath -> NExprLoc -> m (NValue t f m))
-  -> (NValue t f m -> m a)
+  -> (Maybe FilePath -> NExprLoc -> m (NValue f m))
+  -> (NValue f m -> m a)
   -> NExprLoc
   -> m a
 evaluateExpression mpath evaluator handler expr = do
@@ -124,10 +126,10 @@ evaluateExpression mpath evaluator handler expr = do
   argmap args = nvSet (M.fromList args) mempty
 
 processResult
-  :: forall e t f m a
-   . (MonadNix e t f m, Has e Options)
-  => (NValue t f m -> m a)
-  -> NValue t f m
+  :: forall e f m a
+   . (MonadNix e f m, Has e Options)
+  => (NValue f m -> m a)
+  -> NValue f m
   -> m a
 processResult h val = do
   opts :: Options <- asks (view hasLens)
@@ -135,7 +137,7 @@ processResult h val = do
     Nothing                         -> h val
     Just (Text.splitOn "." -> keys) -> go keys val
  where
-  go :: [Text.Text] -> NValue t f m -> m a
+  go :: [Text.Text] -> NValue f m -> m a
   go [] v = h v
   go ((Text.decimal -> Right (n,"")) : ks) v = demand v $ \case
     NVList xs -> case ks of
diff --git a/src/Nix/Builtins.hs b/src/Nix/Builtins.hs
index 3a1b19704..be1bc40a8 100644
--- a/src/Nix/Builtins.hs
+++ b/src/Nix/Builtins.hs
@@ -18,6 +18,8 @@
 {-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE UndecidableInstances #-}
 {-# LANGUAGE ViewPatterns #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE RecursiveDo #-}
 
 {-# OPTIONS_GHC -fno-warn-name-shadowing #-}
 
@@ -27,6 +29,7 @@ module Nix.Builtins (withNixContext, builtins) where
 import           Control.Comonad
 import           Control.Monad
 import           Control.Monad.Catch
+import           Control.Monad.Free
 import           Control.Monad.ListM            ( sortByM )
 import           Control.Monad.Reader           ( asks )
 import           Crypto.Hash
@@ -79,6 +82,7 @@ import           Nix.Scope
 import           Nix.String              hiding (getContext)
 import qualified Nix.String                    as NixString
 import           Nix.String.Coerce
+import           Nix.Thunk
 import           Nix.Utils
 import           Nix.Value
 import           Nix.Value.Equal
@@ -95,8 +99,8 @@ import           Text.Regex.TDFA
 
 -- | Evaluate a nix expression in the default context
 withNixContext
-  :: forall e t f m r
-   . (MonadNix e t f m, Has e Options)
+  :: forall e f m r
+   . (MonadNix e f m, Has e Options)
   => Maybe FilePath
   -> m r
   -> m r
@@ -116,12 +120,14 @@ withNixContext mpath action = do
       let ref = nvPath path
       pushScope (M.singleton "__cur_file" ref) action
 
-builtins :: (MonadNix e t f m, Scoped (NValue t f m) m)
-         => m (Scopes m (NValue t f m))
+builtins :: (MonadNix e f m, Scoped m (NValue f m) m)
+         => m (Scopes m (NValue f m))
 builtins = do
-  ref <- defer $ flip nvSet M.empty <$> buildMap
-  lst <- ([("builtins", ref)] <>) <$> topLevelBuiltins
-  pushScope (M.fromList lst) currentScopes
+  rec lst <- pushScope s $ do
+        ref <- defer $ flip nvSet M.empty <$> buildMap
+        ([("builtins", ref)] <>) <$> topLevelBuiltins
+      let s = M.fromList lst
+  pushScope s currentScopes
  where
   buildMap         = M.fromList . fmap mapping <$> builtinsList
   topLevelBuiltins = fmap mapping <$> fullBuiltinsList
@@ -138,7 +144,7 @@ data Builtin v = Builtin
     , mapping :: (Text, v)
     }
 
-builtinsList :: forall e t f m . MonadNix e t f m => m [Builtin (NValue t f m)]
+builtinsList :: forall e f m . MonadNix e f m => m [Builtin (NValue f m)]
 builtinsList = sequence
   [ do
     version <- toValue (makeNixStringWithoutContext "2.3")
@@ -194,7 +200,7 @@ builtinsList = sequence
   , add  Normal   "getEnv"           getEnv_
   , add2 Normal   "hasAttr"          hasAttr
   , add  Normal   "hasContext"       hasContext
-  , add' Normal   "hashString"       (hashString @e @t @f @m)
+  , add' Normal   "hashString"       (hashString @e @f @m)
   , add  Normal   "head"             head_
   , add  TopLevel "import"           import_
   , add2 Normal   "intersectAttrs"   intersectAttrs
@@ -268,15 +274,15 @@ builtinsList = sequence
   add2 t n v = wrap t n <$> mkThunk n (builtin2 (Text.unpack n) v)
   add3 t n v = wrap t n <$> mkThunk n (builtin3 (Text.unpack n) v)
 
-  add' :: forall a. ToBuiltin t f m a
-       => BuiltinType -> Text -> a -> m (Builtin (NValue t f m))
+  add' :: forall a. ToBuiltin f m a
+       => BuiltinType -> Text -> a -> m (Builtin (NValue f m))
   add' t n v = wrap t n <$> mkThunk n (toBuiltin (Text.unpack n) v)
 
 -- Primops
 
 derivation
-  :: forall e t f m. (MonadNix e t f m, Scoped (NValue t f m) m)
-  => m (NValue t f m)
+  :: forall e f m. (MonadNix e f m, Scoped m (NValue f m) m)
+  => m (NValue f m)
 derivation = foldFix Eval.eval $$(do
     -- This is compiled in so that we only parse it once at compile-time.
     let Success expr = parseNixText [text|
@@ -309,8 +315,8 @@ derivation = foldFix Eval.eval $$(do
   )
 
 foldNixPath
-  :: forall e t f m r
-   . MonadNix e t f m
+  :: forall e f m r
+   . MonadNix e f m
   => (FilePath -> Maybe String -> NixPathEntryType -> r -> m r)
   -> r
   -> m r
@@ -336,7 +342,7 @@ foldNixPath f z = do
     [n, p] -> f (Text.unpack p) (pure (Text.unpack n)) ty rest
     _ -> throwError $ ErrorCall $ "Unexpected entry in NIX_PATH: " <> show x
 
-nixPath :: MonadNix e t f m => m (NValue t f m)
+nixPath :: MonadNix e f m => m (NValue f m)
 nixPath = fmap nvList $ flip foldNixPath mempty $ \p mn ty rest ->
   pure
     $ flip nvSet mempty ( M.fromList
@@ -355,44 +361,44 @@ nixPath = fmap nvList $ flip foldNixPath mempty $ \p mn ty rest ->
       )
     : rest
 
-toString :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
+toString :: MonadNix e f m => NValue f m -> m (NValue f m)
 toString = coerceToString callFunc DontCopyToStore CoerceAny >=> toValue
 
 hasAttr
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 hasAttr x y = fromValue x >>= fromStringNoContext >>= \key ->
-  fromValue @(AttrSet (NValue t f m), AttrSet SourcePos) y
+  fromValue @(AttrSet (NValue f m), AttrSet SourcePos) y
     >>= \(aset, _) -> toValue $ M.member key aset
 
-attrsetGet :: MonadNix e t f m => Text -> AttrSet (NValue t f m) -> m (NValue t f m)
+attrsetGet :: MonadNix e f m => Text -> AttrSet (NValue f m) -> m (NValue f m)
 attrsetGet k s = case M.lookup k s of
   Just v -> pure v
   Nothing ->
     throwError $ ErrorCall $ "Attribute '" <> Text.unpack k <> "' required"
 
-hasContext :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
+hasContext :: MonadNix e f m => NValue f m -> m (NValue f m)
 hasContext = toValue . stringHasContext <=< fromValue
 
 getAttr
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 getAttr x y = fromValue x >>= fromStringNoContext >>= \key ->
-  fromValue @(AttrSet (NValue t f m), AttrSet SourcePos) y
+  fromValue @(AttrSet (NValue f m), AttrSet SourcePos) y
     >>= \(aset, _) -> attrsetGet key aset
 
 unsafeGetAttrPos
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 unsafeGetAttrPos x y = demand x $ \x' -> demand y $ \y' -> case (x', y') of
   (NVStr ns, NVSet _ apos) ->
     case M.lookup (stringIgnoreContext ns) apos of
@@ -407,14 +413,14 @@ unsafeGetAttrPos x y = demand x $ \x' -> demand y $ \y' -> case (x', y') of
 -- This function is a bit special in that it doesn't care about the contents
 -- of the list.
 length_
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
-length_ = toValue . (length :: [NValue t f m] -> Int) <=< fromValue
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
+length_ = toValue . (length :: [NValue f m] -> Int) <=< fromValue
 
 add_
-  :: MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 add_ x y = demand x $ \x' -> demand y $ \y' -> case (x', y') of
   (NVConstant (NInt   x), NVConstant (NInt y)  ) -> toValue (x + y :: Integer)
   (NVConstant (NFloat x), NVConstant (NInt y)  ) -> toValue (x + fromInteger y)
@@ -423,10 +429,10 @@ add_ x y = demand x $ \x' -> demand y $ \y' -> case (x', y') of
   (_                    , _                    ) -> throwError $ Addition x' y'
 
 mul_
-  :: MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 mul_ x y = demand x $ \x' -> demand y $ \y' -> case (x', y') of
   (NVConstant (NInt   x), NVConstant (NInt y)  ) -> toValue (x * y :: Integer)
   (NVConstant (NFloat x), NVConstant (NInt y)  ) -> toValue (x * fromInteger y)
@@ -435,10 +441,10 @@ mul_ x y = demand x $ \x' -> demand y $ \y' -> case (x', y') of
   (_, _) -> throwError $ Multiplication x' y'
 
 div_
-  :: MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 div_ x y = demand x $ \x' -> demand y $ \y' -> case (x', y') of
   (NVConstant (NInt x), NVConstant (NInt y)) | y /= 0 ->
     toValue (floor (fromInteger x / fromInteger y :: Double) :: Integer)
@@ -456,10 +462,10 @@ anyM p (x : xs) = do
   if q then pure True else anyM p xs
 
 any_
-  :: MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 any_ f = toValue <=< anyM fromValue <=< mapM (f `callFunc`) <=< fromValue
 
 allM :: Monad m => (a -> m Bool) -> [a] -> m Bool
@@ -469,28 +475,28 @@ allM p (x : xs) = do
   if q then allM p xs else pure False
 
 all_
-  :: MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 all_ f = toValue <=< allM fromValue <=< mapM (f `callFunc`) <=< fromValue
 
 foldl'_
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
-foldl'_ f z xs = fromValue @[NValue t f m] xs >>= foldM go z
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> NValue f m
+  -> m (NValue f m)
+foldl'_ f z xs = fromValue @[NValue f m] xs >>= foldM go z
   where go b a = f `callFunc` b >>= (`callFunc` a)
 
-head_ :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
+head_ :: MonadNix e f m => NValue f m -> m (NValue f m)
 head_ = fromValue >=> \case
   []    -> throwError $ ErrorCall "builtins.head: empty list"
   h : _ -> pure h
 
-tail_ :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
+tail_ :: MonadNix e f m => NValue f m -> m (NValue f m)
 tail_ = fromValue >=> \case
   []    -> throwError $ ErrorCall "builtins.tail: empty list"
   _ : t -> pure $ nvList t
@@ -535,7 +541,7 @@ splitVersion s = case Text.uncons s of
              x     -> VersionComponent_String x
        in  thisComponent : splitVersion rest
 
-splitVersion_ :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
+splitVersion_ :: MonadNix e f m => NValue f m -> m (NValue f m)
 splitVersion_ = fromValue >=> fromStringNoContext >=> \s ->
   pure
     $ nvList
@@ -552,10 +558,10 @@ compareVersions s1 s2 = mconcat
   f = uncurry compare . fromThese z z
 
 compareVersions_
-  :: MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 compareVersions_ t1 t2 = fromValue t1 >>= fromStringNoContext >>= \s1 ->
   fromValue t2 >>= fromStringNoContext >>= \s2 ->
     pure $ nvConstant $ NInt $ case compareVersions s1 s2 of
@@ -583,10 +589,10 @@ splitDrvName s =
     (Text.intercalate sep namePieces, Text.intercalate sep versionPieces)
 
 parseDrvName
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
 parseDrvName = fromValue >=> fromStringNoContext >=> \s -> do
   let (name :: Text, version :: Text) = splitDrvName s
-  toValue @(AttrSet (NValue t f m)) $ M.fromList
+  toValue @(AttrSet (NValue f m)) $ M.fromList
     [ ( "name" :: Text
       , nvStr $ makeNixStringWithoutContext name
       )
@@ -596,11 +602,11 @@ parseDrvName = fromValue >=> fromStringNoContext >=> \s -> do
     ]
 
 match_
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 match_ pat str = fromValue pat >>= fromStringNoContext >>= \p ->
   fromValue str >>= \ns -> do
         -- NOTE: Currently prim_match in nix/src/libexpr/primops.cc ignores the
@@ -622,11 +628,11 @@ match_ pat str = fromValue pat >>= fromStringNoContext >>= \p ->
       _ -> pure $ nvConstant NNull
 
 split_
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 split_ pat str = fromValue pat >>= fromStringNoContext >>= \p ->
   fromValue str >>= \ns -> do
         -- NOTE: Currently prim_split in nix/src/libexpr/primops.cc ignores the
@@ -641,12 +647,12 @@ split_ pat str = fromValue pat >>= fromStringNoContext >>= \p ->
                                    haystack
 
 splitMatches
-  :: forall e t f m
-   . MonadNix e t f m
+  :: forall e f m
+   . MonadNix e f m
   => Int
   -> [[(ByteString, (Int, Int))]]
   -> ByteString
-  -> [NValue t f m]
+  -> [NValue f m]
 splitMatches _ [] haystack = [thunkStr haystack]
 splitMatches _ ([] : _) _ =
   error "Error in splitMatches: this should never happen!"
@@ -660,10 +666,10 @@ splitMatches numDropped (((_, (start, len)) : captures) : mts) haystack =
   caps           = nvList (fmap f captures)
   f (a, (s, _)) = if s < 0 then nvConstant NNull else thunkStr a
 
-thunkStr :: Applicative f => ByteString -> NValue t f m
+thunkStr :: Applicative f => ByteString -> NValue f m
 thunkStr s = nvStr (makeNixStringWithoutContext (decodeUtf8 s))
 
-substring :: forall e t f m. MonadNix e t f m => Int -> Int -> NixString -> Prim m NixString
+substring :: forall e f m. MonadNix e f m => Int -> Int -> NixString -> Prim m NixString
 substring start len str = Prim $
   if start < 0
   then throwError $ ErrorCall $ "builtins.substring: negative start position: " <> show start
@@ -673,9 +679,9 @@ substring start len str = Prim $
   take = if len < 0 then id else Text.take len
 
 attrNames
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
 attrNames =
-  fromValue @(AttrSet (NValue t f m))
+  fromValue @(AttrSet (NValue f m))
     >=> fmap getDeeper
     .   toValue
     .   fmap makeNixStringWithoutContext
@@ -683,108 +689,108 @@ attrNames =
     .   M.keys
 
 attrValues
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
 attrValues =
-  fromValue @(AttrSet (NValue t f m))
+  fromValue @(AttrSet (NValue f m))
     >=> toValue
     .   fmap snd
-    .   sortOn (fst @Text @(NValue t f m))
+    .   sortOn (fst @Text @(NValue f m))
     .   M.toList
 
 map_
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 map_ f =
   toValue
     <=< traverse
-          ( defer @(NValue t f m)
+          ( defer @(NValue f m)
           . withFrame Debug (ErrorCall "While applying f in map:\n")
           . (f `callFunc`)
           )
-    <=< fromValue @[NValue t f m]
+    <=< fromValue @[NValue f m]
 
 mapAttrs_
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
-mapAttrs_ f xs = fromValue @(AttrSet (NValue t f m)) xs >>= \aset -> do
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
+mapAttrs_ f xs = fromValue @(AttrSet (NValue f m)) xs >>= \aset -> do
   let pairs = M.toList aset
   values <- for pairs $ \(key, value) ->
-    defer @(NValue t f m)
+    defer @(NValue f m)
       $   withFrame Debug (ErrorCall "While applying f in mapAttrs:\n")
       $   callFunc ?? value
       =<< callFunc f (nvStr (makeNixStringWithoutContext key))
   toValue . M.fromList . zip (fmap fst pairs) $ values
 
 filter_
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 filter_ f =
   toValue
     <=< filterM (fromValue <=< callFunc f)
     <=< fromValue
 
 catAttrs
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 catAttrs attrName xs = fromValue attrName >>= fromStringNoContext >>= \n ->
-  fromValue @[NValue t f m] xs >>= \l ->
+  fromValue @[NValue f m] xs >>= \l ->
     fmap (nvList . catMaybes)
       $ forM l
       $ fmap (M.lookup n)
       . flip demand fromValue
 
-baseNameOf :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
+baseNameOf :: MonadNix e f m => NValue f m -> m (NValue f m)
 baseNameOf x = do
   ns <- coerceToString callFunc DontCopyToStore CoerceStringy x
   pure $ nvStr
     (modifyNixContents (Text.pack . takeFileName . Text.unpack) ns)
 
 bitAnd
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 bitAnd x y =
   fromValue @Integer x >>= \a -> fromValue @Integer y >>= \b -> toValue (a .&. b)
 
 bitOr
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 bitOr x y =
   fromValue @Integer x >>= \a -> fromValue @Integer y >>= \b -> toValue (a .|. b)
 
 bitXor
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 bitXor x y = fromValue @Integer x
   >>= \a -> fromValue @Integer y >>= \b -> toValue (a `xor` b)
 
 builtinsBuiltin
-  :: forall e t f m
-   . MonadNix e t f m
-  => m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => m (NValue f m)
 builtinsBuiltin = (throwError $ ErrorCall "HNix does not provide builtins.builtins at the moment. Using builtins directly should be preferred")
 
-dirOf :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
+dirOf :: MonadNix e f m => NValue f m -> m (NValue f m)
 dirOf x = demand x $ \case
   NVStr ns -> pure $ nvStr
     (modifyNixContents (Text.pack . takeDirectory . Text.unpack) ns)
@@ -794,33 +800,33 @@ dirOf x = demand x $ \case
 
 -- jww (2018-04-28): This should only be a string argument, and not coerced?
 unsafeDiscardStringContext
-  :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
+  :: MonadNix e f m => NValue f m -> m (NValue f m)
 unsafeDiscardStringContext mnv = do
   ns <- fromValue mnv
   toValue $ makeNixStringWithoutContext $ stringIgnoreContext
     ns
 
 seq_
-  :: MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 seq_ a b = demand a $ \_ -> pure b
 
 -- | We evaluate 'a' only for its effects, so data cycles are ignored.
 deepSeq
-  :: MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 deepSeq a b = b <$ normalForm_ a
 
 elem_
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 elem_ x = toValue <=< anyM (valueEqM x) <=< fromValue
 
 elemAt :: [a] -> Int -> Maybe a
@@ -829,10 +835,10 @@ elemAt ls i = case drop i ls of
   a : _ -> pure a
 
 elemAt_
-  :: MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 elemAt_ xs n = fromValue n >>= \n' -> fromValue xs >>= \xs' ->
   case elemAt xs' n' of
     Just a -> pure a
@@ -845,11 +851,11 @@ elemAt_ xs n = fromValue n >>= \n' -> fromValue xs >>= \xs' ->
         <> show (length xs')
 
 genList
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 genList f = fromValue @Integer >=> \n -> if n >= 0
   then toValue =<< forM [0 .. n - 1] (\i -> defer $ (f `callFunc`) =<< toValue i)
   else
@@ -859,9 +865,9 @@ genList f = fromValue @Integer >=> \n -> if n >= 0
     <> show n
 
 -- We wrap values solely to provide an Ord instance for genericClosure
-newtype WValue t f m = WValue (NValue t f m)
+newtype WValue f m = WValue (NValue f m)
 
-instance Comonad f => Eq (WValue t f m) where
+instance Comonad f => Eq (WValue f m) where
   WValue (NVConstant (NFloat x)) == WValue (NVConstant (NInt y)) =
     x == fromInteger y
   WValue (NVConstant (NInt   x)) == WValue (NVConstant (NFloat y)) =
@@ -873,7 +879,7 @@ instance Comonad f => Eq (WValue t f m) where
     stringIgnoreContext x == stringIgnoreContext y
   _ == _ = False
 
-instance Comonad f => Ord (WValue t f m) where
+instance Comonad f => Ord (WValue f m) where
   WValue (NVConstant (NFloat x)) <= WValue (NVConstant (NInt y)) =
     x <= fromInteger y
   WValue (NVConstant (NInt   x)) <= WValue (NVConstant (NFloat y)) =
@@ -886,8 +892,8 @@ instance Comonad f => Ord (WValue t f m) where
   _ <= _ = False
 
 genericClosure
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
-genericClosure = fromValue @(AttrSet (NValue t f m)) >=> \s ->
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
+genericClosure = fromValue @(AttrSet (NValue f m)) >=> \s ->
   case (M.lookup "startSet" s, M.lookup "operator" s) of
     (Nothing, Nothing) ->
       throwError
@@ -903,22 +909,22 @@ genericClosure = fromValue @(AttrSet (NValue t f m)) >=> \s ->
         $ ErrorCall
         $ "builtins.genericClosure: Attribute 'operator' required"
     (Just startSet, Just operator) ->
-      demand startSet $ fromValue @[NValue t f m] >=> \ss ->
-        demand operator $ \op -> toValue @[NValue t f m] =<< snd <$> go op ss S.empty
+      demand startSet $ fromValue @[NValue f m] >=> \ss ->
+        demand operator $ \op -> toValue @[NValue f m] =<< snd <$> go op ss S.empty
  where
   go
-    :: NValue t f m
-    -> [NValue t f m]
-    -> Set (WValue t f m)
-    -> m (Set (WValue t f m), [NValue t f m])
+    :: NValue f m
+    -> [NValue f m]
+    -> Set (WValue f m)
+    -> m (Set (WValue f m), [NValue f m])
   go _  []       ks = pure (ks, mempty)
-  go op (t : ts) ks = demand t $ \v -> fromValue @(AttrSet (NValue t f m)) v >>= \s -> do
+  go op (t : ts) ks = demand t $ \v -> fromValue @(AttrSet (NValue f m)) v >>= \s -> do
     k <- attrsetGet "key" s
     demand k $ \k' -> do
       if S.member (WValue k') ks
         then go op ts ks
         else do
-          ys <- fromValue @[NValue t f m] =<< (op `callFunc` v)
+          ys <- fromValue @[NValue f m] =<< (op `callFunc` v)
           case S.toList ks of
             []           -> checkComparable k' k'
             WValue j : _ -> checkComparable k' j
@@ -933,11 +939,11 @@ genericClosure = fromValue @(AttrSet (NValue t f m)) >=> \s ->
 -- Example:
 -- builtins.replaceStrings ["ll" "e"] [" " "i"] "Hello world" == "Hi o world".
 replaceStrings
-  :: MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 replaceStrings tfrom tto ts =
   do
     -- NixStrings have context - remember
@@ -1016,33 +1022,33 @@ replaceStrings tfrom tto ts =
     toValue $ go (NixString.getContext string) (stringIgnoreContext string) mempty
 
 removeAttrs
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 removeAttrs set = fromValue . Deeper >=> \(nsToRemove :: [NixString]) ->
-  fromValue @(AttrSet (NValue t f m), AttrSet SourcePos) set >>= \(m, p) -> do
+  fromValue @(AttrSet (NValue f m), AttrSet SourcePos) set >>= \(m, p) -> do
     toRemove <- mapM fromStringNoContext nsToRemove
     toValue (go m toRemove, go p toRemove)
   where go = foldl' (flip M.delete)
 
 intersectAttrs
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 intersectAttrs set1 set2 =
-  fromValue @(AttrSet (NValue t f m), AttrSet SourcePos) set1 >>= \(s1, p1) ->
-    fromValue @(AttrSet (NValue t f m), AttrSet SourcePos) set2 >>= \(s2, p2) ->
+  fromValue @(AttrSet (NValue f m), AttrSet SourcePos) set1 >>= \(s1, p1) ->
+    fromValue @(AttrSet (NValue f m), AttrSet SourcePos) set2 >>= \(s2, p2) ->
       pure $ nvSet (s2 `M.intersection` s1) (p2 `M.intersection` p1)
 
 functionArgs
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
 functionArgs fun = demand fun $ \case
   NVClosure p _ ->
-    toValue @(AttrSet (NValue t f m)) $ nvConstant . NBool <$> case p of
+    toValue @(AttrSet (NValue f m)) $ nvConstant . NBool <$> case p of
       Param name     -> M.singleton name False
       ParamSet s _ _ -> isJust <$> M.fromList s
   v ->
@@ -1052,10 +1058,10 @@ functionArgs fun = demand fun $ \case
       <> show v
 
 toFile
-  :: MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 toFile name s = do
   name' <- fromStringNoContext =<< fromValue name
   s'    <- fromValue s
@@ -1065,10 +1071,10 @@ toFile name s = do
       sc = StringContext t DirectPath
   toValue $ makeNixStringWithSingletonContext t sc
 
-toPath :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
+toPath :: MonadNix e f m => NValue f m -> m (NValue f m)
 toPath = fromValue @Path >=> toValue @Path
 
-pathExists_ :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
+pathExists_ :: MonadNix e f m => NValue f m -> m (NValue f m)
 pathExists_ path = demand path $ \case
   NVPath p  -> toValue =<< pathExists p
   NVStr  ns -> toValue =<< pathExists (Text.unpack (stringIgnoreContext ns))
@@ -1079,67 +1085,67 @@ pathExists_ path = demand path $ \case
       <> show v
 
 hasKind
-  :: forall a e t f m
-   . (MonadNix e t f m, FromValue a m (NValue t f m))
-  => NValue t f m
-  -> m (NValue t f m)
+  :: forall a e f m
+   . (MonadNix e f m, FromValue a m (NValue f m))
+  => NValue f m
+  -> m (NValue f m)
 hasKind = fromValueMay >=> toValue . \case
   Just (_ :: a) -> True
   _             -> False
 
 isAttrs
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
-isAttrs = hasKind @(AttrSet (NValue t f m))
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
+isAttrs = hasKind @(AttrSet (NValue f m))
 
 isList
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
-isList = hasKind @[NValue t f m]
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
+isList = hasKind @[NValue f m]
 
 isInt
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
 isInt = hasKind @Int
 
 isFloat
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
 isFloat = hasKind @Float
 
 isBool
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
 isBool = hasKind @Bool
 
 isNull
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
 isNull = hasKind @()
 
 -- isString cannot use `hasKind` because it coerces derivations to strings.
-isString :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
+isString :: MonadNix e f m => NValue f m -> m (NValue f m)
 isString v = demand v $ \case
   NVStr{} -> toValue True
   _       -> toValue False
 
-isFunction :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
+isFunction :: MonadNix e f m => NValue f m -> m (NValue f m)
 isFunction func = demand func $ \case
   NVClosure{} -> toValue True
   _           -> toValue False
 
-throw_ :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
+throw_ :: MonadNix e f m => NValue f m -> m (NValue f m)
 throw_ mnv = do
   ns <- coerceToString callFunc CopyToStore CoerceStringy mnv
   throwError . ErrorCall . Text.unpack $ stringIgnoreContext ns
 
 import_
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
 import_ = scopedImport (nvSet M.empty M.empty)
 
 scopedImport
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
-scopedImport asetArg pathArg = fromValue @(AttrSet (NValue t f m)) asetArg >>= \s ->
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
+scopedImport asetArg pathArg = fromValue @(AttrSet (NValue f m)) asetArg >>= \s ->
   fromValue pathArg >>= \(Path p) -> do
-    path  <- pathToDefaultNix @t @f @m p
+    path  <- pathToDefaultNix @f @m p
     mres  <- lookupVar "__cur_file"
     path' <- case mres of
       Nothing -> do
@@ -1148,21 +1154,21 @@ scopedImport asetArg pathArg = fromValue @(AttrSet (NValue t f m)) asetArg >>= \
       Just p -> demand p $ fromValue >=> \(Path p') -> do
         traceM $ "Current file being evaluated is: " <> show p'
         pure $ takeDirectory p' </> path
-    clearScopes @(NValue t f m)
+    clearScopes
       $ withNixContext (pure path')
       $ pushScope s
-      $ importPath @t @f @m path'
+      $ importPath @f @m path'
 
-getEnv_ :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
+getEnv_ :: MonadNix e f m => NValue f m -> m (NValue f m)
 getEnv_ = fromValue >=> fromStringNoContext >=> \s -> do
   mres <- getEnvVar (Text.unpack s)
   toValue $ makeNixStringWithoutContext $ maybe "" Text.pack mres
 
 sort_
-  :: MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 sort_ comp = fromValue >=> sortByM (cmp comp) >=> toValue
  where
   cmp f a b = do
@@ -1176,10 +1182,10 @@ sort_ comp = fromValue >=> sortByM (cmp comp) >=> toValue
           False -> EQ
 
 lessThan
-  :: MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 lessThan ta tb = demand ta $ \va -> demand tb $ \vb -> do
   let badType =
         throwError
@@ -1201,34 +1207,34 @@ lessThan ta tb = demand ta $ \va -> demand tb $ \vb -> do
     _ -> badType
 
 concatLists
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
 concatLists =
-  fromValue @[NValue t f m]
-    >=> mapM (flip demand $ fromValue @[NValue t f m] >=> pure)
+  fromValue @[NValue f m]
+    >=> mapM (flip demand $ fromValue @[NValue f m] >=> pure)
     >=> toValue
     .   concat
 
 concatMap_
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 concatMap_ f =
-  fromValue @[NValue t f m]
+  fromValue @[NValue f m]
     >=> traverse applyFunc
     >=> toValue . concat
   where
-    applyFunc :: NValue t f m  -> m [NValue t f m]
+    applyFunc :: NValue f m  -> m [NValue f m]
     applyFunc =  (f `callFunc`) >=> fromValue
 
 listToAttrs
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
-listToAttrs = fromValue @[NValue t f m] >=> \l ->
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
+listToAttrs = fromValue @[NValue f m] >=> \l ->
   fmap (flip nvSet M.empty . M.fromList . reverse)
     $   forM l
     $   flip demand
-    $   fromValue @(AttrSet (NValue t f m))
+    $   fromValue @(AttrSet (NValue f m))
     >=> \s -> do
           t <- attrsetGet "name" s
           demand t $ fromValue >=> \n -> do
@@ -1241,7 +1247,7 @@ listToAttrs = fromValue @[NValue t f m] >=> \l ->
 -- propagate context from the s arg
 -- | The result coming out of hashString is base16 encoded
 hashString
-  :: forall e t f m. MonadNix e t f m => NixString -> NixString -> Prim m NixString
+  :: forall e f m. MonadNix e f m => NixString -> NixString -> Prim m NixString
 hashString nsAlgo ns = Prim $ do
   algo <- fromStringNoContext nsAlgo
   let f g = pure $ modifyNixContents g ns
@@ -1265,7 +1271,7 @@ hashString nsAlgo ns = Prim $ do
         <> "expected \"md5\", \"sha1\", \"sha256\", or \"sha512\", got "
         <> show algo
 
-placeHolder :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
+placeHolder :: MonadNix e f m => NValue f m -> m (NValue f m)
 placeHolder = fromValue >=> fromStringNoContext >=> \t -> do
   h <- runPrim
     (hashString (makeNixStringWithoutContext "sha256")
@@ -1286,7 +1292,7 @@ placeHolder = fromValue >=> fromStringNoContext >=> \t -> do
    where
     text h = encodeUtf8 $ stringIgnoreContext h
 
-absolutePathFromValue :: MonadNix e t f m => NValue t f m -> m FilePath
+absolutePathFromValue :: MonadNix e f m => NValue f m -> m FilePath
 absolutePathFromValue = \case
   NVStr ns -> do
     let path = Text.unpack $ stringIgnoreContext ns
@@ -1300,20 +1306,20 @@ absolutePathFromValue = \case
   NVPath path -> pure path
   v           -> throwError $ ErrorCall $ "expected a path, got " <> show v
 
-readFile_ :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
+readFile_ :: MonadNix e f m => NValue f m -> m (NValue f m)
 readFile_ path = demand path $
   absolutePathFromValue >=> Nix.Render.readFile >=> toValue
 
 findFile_
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 findFile_ aset filePath = demand aset $ \aset' -> demand filePath $ \filePath' ->
   case (aset', filePath') of
     (NVList x, NVStr ns) -> do
-      mres <- findPath @t @f @m x (Text.unpack (stringIgnoreContext ns))
+      mres <- findPath @f @m x (Text.unpack (stringIgnoreContext ns))
       pure $ nvPath mres
     (NVList _, y) ->
       throwError $ ErrorCall $ "expected a string, got " <> show y
@@ -1329,7 +1335,7 @@ data FileType
    | FileTypeUnknown
    deriving (Show, Read, Eq, Ord)
 
-instance Convertible e t f m => ToValue FileType m (NValue t f m) where
+instance (Convertible e t f m, t ~ Thunk m) => ToValue FileType m (Free (NValue' f m) t) where
   toValue = toValue . makeNixStringWithoutContext . \case
     FileTypeRegular   -> "regular" :: Text
     FileTypeDirectory -> "directory"
@@ -1337,7 +1343,7 @@ instance Convertible e t f m => ToValue FileType m (NValue t f m) where
     FileTypeUnknown   -> "unknown"
 
 readDir_
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
 readDir_ p = demand p $ \path' -> do
   path           <- absolutePathFromValue path'
   items          <- listDirectory path
@@ -1352,7 +1358,7 @@ readDir_ p = demand p $ \path' -> do
   getDeeper <$> toValue (M.fromList itemsWithTypes)
 
 fromJSON
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
 fromJSON arg = demand arg $ fromValue >=> fromStringNoContext >=> \encoded ->
   case A.eitherDecodeStrict' @A.Value $ encodeUtf8 encoded of
     Left jsonError ->
@@ -1369,13 +1375,13 @@ fromJSON arg = demand arg $ fromValue >=> fromStringNoContext >=> \encoded ->
     A.Bool   b -> pure $ nvConstant $ NBool b
     A.Null     -> pure $ nvConstant NNull
 
-prim_toJSON :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
+prim_toJSON :: MonadNix e f m => NValue f m -> m (NValue f m)
 prim_toJSON x = demand x $ fmap nvStr . nvalueToJSONNixString
 
-toXML_ :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
+toXML_ :: MonadNix e f m => NValue f m -> m (NValue f m)
 toXML_ v = demand v $ fmap (nvStr . toXML) . normalForm
 
-typeOf :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
+typeOf :: MonadNix e f m => NValue f m -> m (NValue f m)
 typeOf v = demand v $ toValue . makeNixStringWithoutContext . \case
   NVConstant a -> case a of
     NURI   _ -> "string"
@@ -1392,26 +1398,26 @@ typeOf v = demand v $ toValue . makeNixStringWithoutContext . \case
   _             -> error "Pattern synonyms obscure complete patterns"
 
 tryEval
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
 tryEval e = catch (demand e (pure . onSuccess)) (pure . onError)
  where
   onSuccess v = flip nvSet M.empty $ M.fromList
     [("success", nvConstant (NBool True)), ("value", v)]
 
-  onError :: SomeException -> NValue t f m
+  onError :: SomeException -> NValue f m
   onError _ = flip nvSet M.empty $ M.fromList
     [ ("success", nvConstant (NBool False))
     , ("value"  , nvConstant (NBool False))
     ]
 
 trace_
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 trace_ msg action = do
-  traceEffect @t @f @m
+  traceEffect @f @m
     .   Text.unpack
     .   stringIgnoreContext
     =<< fromValue msg
@@ -1419,17 +1425,17 @@ trace_ msg action = do
 
 -- TODO: remember error context
 addErrorContext
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 addErrorContext _ action = pure action
 
 exec_
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
 exec_ xs = do
-  ls <- fromValue @[NValue t f m] xs
+  ls <- fromValue @[NValue f m] xs
   xs <- traverse (coerceToString callFunc DontCopyToStore CoerceStringy) ls
   -- TODO Still need to do something with the context here
   -- See prim_exec in nix/src/libexpr/primops.cc
@@ -1437,7 +1443,7 @@ exec_ xs = do
   exec (fmap (Text.unpack . stringIgnoreContext) xs)
 
 fetchurl
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
 fetchurl v = demand v $ \case
   NVSet s _ -> attrsetGet "url" s >>= demand ?? go (M.lookup "sha256" s)
   v@NVStr{} -> go Nothing v
@@ -1447,7 +1453,7 @@ fetchurl v = demand v $ \case
       $  "builtins.fetchurl: Expected URI or set, got "
       <> show v
  where
-  go :: Maybe (NValue t f m) -> NValue t f m -> m (NValue t f m)
+  go :: Maybe (NValue f m) -> NValue f m -> m (NValue f m)
   go _msha = \case
     NVStr ns -> noContextAttrs ns >>= getURL >>= \case -- msha
       Left  e -> throwError e
@@ -1464,53 +1470,53 @@ fetchurl v = demand v $ \case
     Just t -> pure t
 
 partition_
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
-partition_ f = fromValue @[NValue t f m] >=> \l -> do
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
+partition_ f = fromValue @[NValue f m] >=> \l -> do
   let match t = f `callFunc` t >>= fmap (, t) . fromValue
   selection <- traverse match l
   let (right, wrong) = partition fst selection
   let makeSide       = nvList . fmap snd
-  toValue @(AttrSet (NValue t f m))
+  toValue @(AttrSet (NValue f m))
     $ M.fromList [("right", makeSide right), ("wrong", makeSide wrong)]
 
-currentSystem :: MonadNix e t f m => m (NValue t f m)
+currentSystem :: MonadNix e f m => m (NValue f m)
 currentSystem = do
   os   <- getCurrentSystemOS
   arch <- getCurrentSystemArch
   pure $ nvStr $ makeNixStringWithoutContext (arch <> "-" <> os)
 
-currentTime_ :: MonadNix e t f m => m (NValue t f m)
+currentTime_ :: MonadNix e f m => m (NValue f m)
 currentTime_ = do
   opts :: Options <- asks (view hasLens)
   toValue @Integer $ round $ Time.utcTimeToPOSIXSeconds (currentTime opts)
 
-derivationStrict_ :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
+derivationStrict_ :: MonadNix e f m => NValue f m -> m (NValue f m)
 derivationStrict_ = derivationStrict
 
-getRecursiveSize :: (MonadIntrospect m, Applicative f) => a -> m (NValue t f m)
+getRecursiveSize :: (MonadIntrospect m, Applicative f) => a -> m (NValue f m)
 getRecursiveSize = fmap (nvConstant . NInt . fromIntegral) . recursiveSize
 
 getContext
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
 getContext x = demand x $ \case
   (NVStr ns) -> do
     let context =
           getNixLikeContext $ toNixLikeContext $ NixString.getContext ns
-    valued :: M.HashMap Text (NValue t f m) <- sequenceA $ M.map toValue context
+    valued :: M.HashMap Text (NValue f m) <- sequenceA $ M.map toValue context
     pure $ nvSet valued M.empty
   x ->
     throwError $ ErrorCall $ "Invalid type for builtins.getContext: " <> show x
 
 appendContext
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 appendContext x y = demand x $ \x' -> demand y $ \y' -> case (x', y') of
   (NVStr ns, NVSet attrs _) -> do
     newContextValues <- forM attrs $ \attr -> demand attr $ \case
@@ -1553,17 +1559,17 @@ appendContext x y = demand x $ \x' -> demand y $ \y' -> case (x', y') of
 newtype Prim m a = Prim { runPrim :: m a }
 
 -- | Types that support conversion to nix in a particular monad
-class ToBuiltin t f m a | a -> m where
-  toBuiltin :: String -> a -> m (NValue t f m)
+class ToBuiltin f m a | a -> m where
+  toBuiltin :: String -> a -> m (NValue f m)
 
-instance (MonadNix e t f m, ToValue a m (NValue t f m))
-         => ToBuiltin t f m (Prim m a) where
+instance (MonadNix e f m, ToValue a m (NValue f m))
+        => ToBuiltin f m (Prim m a) where
   toBuiltin _ p = toValue =<< runPrim p
 
-instance ( MonadNix e t f m
-         , FromValue a m (Deeper (NValue t f m))
-         , ToBuiltin t f m b
+instance ( MonadNix e f m
+         , FromValue a m (Deeper (NValue f m))
+         , ToBuiltin f m b
          )
-         => ToBuiltin t f m (a -> b) where
+         => ToBuiltin f m (a -> b) where
   toBuiltin name f =
     pure $ nvBuiltin name (fromValue . Deeper >=> toBuiltin name . f)
diff --git a/src/Nix/Cited.hs b/src/Nix/Cited.hs
index 3e94430e5..bdfd46468 100644
--- a/src/Nix/Cited.hs
+++ b/src/Nix/Cited.hs
@@ -18,7 +18,7 @@ import           Lens.Family2.TH
 
 import           Nix.Expr.Types.Annotated
 import           Nix.Scope
-import           Nix.Value                      ( NValue, NValue'(NValue) )
+import           Nix.Value
 import           Control.Monad.Free             ( Free(Pure, Free) )
 
 data Provenance m v = Provenance
@@ -51,6 +51,7 @@ instance ComonadEnv [Provenance m v] (NCited m v) where
 $(makeLenses ''Provenance)
 $(makeLenses ''NCited)
 
+
 class HasCitations m v a where
     citations :: a -> [Provenance m v]
     addProvenance :: Provenance m v -> a -> a
@@ -59,17 +60,18 @@ instance HasCitations m v (NCited m v a) where
   citations = _provenance
   addProvenance x (NCited p v) = NCited (x : p) v
 
+
 class HasCitations1 m v f where
     citations1 :: f a -> [Provenance m v]
     addProvenance1 :: Provenance m v -> f a -> f a
 
 instance HasCitations1 m v f
-  => HasCitations m v (NValue' t f m a) where
+  => HasCitations m v (NValue' f m a) where
   citations (NValue f) = citations1 f
   addProvenance x (NValue f) = NValue (addProvenance1 x f)
 
 instance (HasCitations1 m v f, HasCitations m v t)
-  => HasCitations m v (NValue t f m) where
+  => HasCitations m v (Free (NValue' f m) t) where
   citations (Pure t) = citations t
   citations (Free v) = citations v
   addProvenance x (Pure t) = Pure (addProvenance x t)
diff --git a/src/Nix/Cited/Basic.hs b/src/Nix/Cited/Basic.hs
index 935b8ecd1..edb9c493e 100644
--- a/src/Nix/Cited/Basic.hs
+++ b/src/Nix/Cited/Basic.hs
@@ -5,29 +5,24 @@
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE ScopedTypeVariables #-}
-{-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE StandaloneDeriving #-}
+{-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE UndecidableInstances #-}
-{-# LANGUAGE ViewPatterns #-}
 
 module Nix.Cited.Basic where
 
 import           Control.Comonad                ( Comonad )
 import           Control.Comonad.Env            ( ComonadEnv )
-import           Control.Monad.Catch     hiding ( catchJust )
-import           Control.Monad.Reader
-import           Data.Fix
+import           Control.Monad.Free
 import           GHC.Generics
 import           Nix.Cited
-import           Nix.Eval                      as Eval
-import           Nix.Exec
-import           Nix.Expr
 import           Nix.Frames
-import           Nix.Options
 import           Nix.Thunk
-import           Nix.Utils
 import           Nix.Value
 
-newtype Cited t f m a = Cited { getCited :: NCited m (NValue t f m) a }
+newtype CitedT (f :: * -> *) m a = CitedT { unCitedT :: m a }
+
+newtype Cited f m a = Cited { getCited :: NCited m (NValue f m) a }
   deriving
     ( Generic
     , Typeable
@@ -36,25 +31,29 @@ newtype Cited t f m a = Cited { getCited :: NCited m (NValue t f m) a }
     , Foldable
     , Traversable
     , Comonad
-    , ComonadEnv [Provenance m (NValue t f m)]
     )
 
-instance HasCitations1 m (NValue t f m) (Cited t f m) where
+deriving instance t ~ Thunk m => ComonadEnv [Provenance m (Free (NValue' f m) t)] (Cited f m)
+
+instance t ~ Thunk m => HasCitations1 m (Free (NValue' f m) t) (Cited f m) where
   citations1 (Cited c) = citations c
   addProvenance1 x (Cited c) = Cited (addProvenance x c)
 
+{-
 instance ( Has e Options
          , Framed e m
-         , MonadThunk t m v
+         , MonadThunk m
          , Typeable m
          , Typeable f
-         , Typeable u
          , MonadCatch m
          )
-  => MonadThunk (Cited u f m t) m v where
+  => MonadThunk (CitedT f m) where
+  type Thunk (CitedT f m) = Cited f m (Thunk m)
+  type ThunkValue (CitedT f m) = ThunkValue m
   thunk mv = do
     opts :: Options <- asks (view hasLens)
 
+    --TODO: Can we handle `thunks opts == false` by not using CitedT at all?
     if thunks opts
       then do
         frames :: Frames <- asks (view hasLens)
@@ -69,11 +68,9 @@ instance ( Has e Options
             go _ = mempty
             ps = concatMap (go . frame) frames
 
-        fmap (Cited . NCited ps) . thunk $ mv
+        lift $ fmap (Cited . NCited ps) . thunk $ mv
       else fmap (Cited . NCited mempty) . thunk $ mv
 
-  thunkId (Cited (NCited _ t)) = thunkId @_ @m t
-
   queryM (Cited (NCited _ t)) = queryM t
 
   -- | The ThunkLoop exception is thrown as an exception with MonadThrow,
@@ -98,3 +95,4 @@ instance ( Has e Options
         withFrame Info (ForcingExpr scope (wrapExprLoc s e)) (forceEff t f)
 
   further (Cited (NCited ps t)) f = Cited . NCited ps <$> further t f
+-}
diff --git a/src/Nix/Context.hs b/src/Nix/Context.hs
index fec3caff0..f8466f84d 100644
--- a/src/Nix/Context.hs
+++ b/src/Nix/Context.hs
@@ -5,31 +5,27 @@
 module Nix.Context where
 
 import           Nix.Options
-import           Nix.Scope
 import           Nix.Frames
 import           Nix.Utils
 import           Nix.Expr.Types.Annotated       ( SrcSpan
                                                 , nullSpan
                                                 )
 
-data Context m t = Context
-    { scopes  :: Scopes m t
-    , source  :: SrcSpan
-    , frames  :: Frames
-    , options :: Options
+data Context = Context
+    { source  :: SrcSpan -- Should we capture?
+    , frames  :: Frames -- Don't capture (should change)
+    , options :: Options -- Don't capture (never changes)
     }
 
-instance Has (Context m t) (Scopes m t) where
-  hasLens f a = (\x -> a { scopes = x }) <$> f (scopes a)
 
-instance Has (Context m t) SrcSpan where
+instance Has Context SrcSpan where
   hasLens f a = (\x -> a { source = x }) <$> f (source a)
 
-instance Has (Context m t) Frames where
+instance Has Context Frames where
   hasLens f a = (\x -> a { frames = x }) <$> f (frames a)
 
-instance Has (Context m t) Options where
+instance Has Context Options where
   hasLens f a = (\x -> a { options = x }) <$> f (options a)
 
-newContext :: Options -> Context m t
-newContext = Context emptyScopes nullSpan mempty
+newContext :: Options -> Context
+newContext = Context nullSpan mempty
diff --git a/src/Nix/Convert.hs b/src/Nix/Convert.hs
index 9e2ec0d9e..a9569af0d 100644
--- a/src/Nix/Convert.hs
+++ b/src/Nix/Convert.hs
@@ -69,13 +69,13 @@ class FromValue a m v where
     fromValueMay :: v -> m (Maybe a)
 
 type Convertible e t f m
-  = (Framed e m, MonadDataErrorContext t f m, MonadThunk t m (NValue t f m))
+  = (Framed e m, MonadDataErrorContext f m, MonadThunk m, Thunk m ~ t, ThunkValue m ~ NValue f m)
 
 instance ( Convertible e t f m
-         , MonadValue (NValue t f m) m
-         , FromValue a m (NValue' t f m (NValue t f m))
+         , MonadValue (NValue f m) m
+         , FromValue a m (NValue' f m (NValue f m))
          )
-  => FromValue a m (NValue t f m) where
+  => FromValue a m (Free (NValue' f m) t) where
   fromValueMay = flip demand $ \case
     Pure t -> force t fromValueMay
     Free v -> fromValueMay v
@@ -84,10 +84,10 @@ instance ( Convertible e t f m
     Free v -> fromValue v
 
 instance ( Convertible e t f m
-         , MonadValue (NValue t f m) m
-         , FromValue a m (Deeper (NValue' t f m (NValue t f m)))
+         , MonadValue (NValue f m) m
+         , FromValue a m (Deeper (NValue' f m (NValue f m)))
          )
-  => FromValue a m (Deeper (NValue t f m)) where
+  => FromValue a m (Deeper (Free (NValue' f m) t)) where
   fromValueMay (Deeper v) = demand v $ \case
     Pure t -> force t (fromValueMay . Deeper)
     Free v -> fromValueMay (Deeper v)
@@ -96,56 +96,56 @@ instance ( Convertible e t f m
     Free v -> fromValue (Deeper v)
 
 instance Convertible e t f m
-  => FromValue () m (NValue' t f m (NValue t f m)) where
+  => FromValue () m (NValue' f m (Free (NValue' f m) t)) where
   fromValueMay = \case
     NVConstant' NNull -> pure $ pure ()
     _                 -> pure mempty
   fromValue v = fromValueMay v >>= \case
     Just b -> pure b
-    _      -> throwError $ Expectation @t @f @m TNull (Free v)
+    _      -> throwError $ Expectation @f @m TNull (Free v)
 
 instance Convertible e t f m
-  => FromValue Bool m (NValue' t f m (NValue t f m)) where
+  => FromValue Bool m (NValue' f m (Free (NValue' f m) t)) where
   fromValueMay = \case
     NVConstant' (NBool b) -> pure $ pure b
     _                     -> pure Nothing
   fromValue v = fromValueMay v >>= \case
     Just b -> pure b
-    _      -> throwError $ Expectation @t @f @m TBool (Free v)
+    _      -> throwError $ Expectation @f @m TBool (Free v)
 
 instance Convertible e t f m
-  => FromValue Int m (NValue' t f m (NValue t f m)) where
+  => FromValue Int m (NValue' f m (Free (NValue' f m) t)) where
   fromValueMay = \case
     NVConstant' (NInt b) -> pure $ pure (fromInteger b)
     _                    -> pure Nothing
   fromValue v = fromValueMay v >>= \case
     Just b -> pure b
-    _      -> throwError $ Expectation @t @f @m TInt (Free v)
+    _      -> throwError $ Expectation @f @m TInt (Free v)
 
 instance Convertible e t f m
-  => FromValue Integer m (NValue' t f m (NValue t f m)) where
+  => FromValue Integer m (NValue' f m (Free (NValue' f m) t)) where
   fromValueMay = \case
     NVConstant' (NInt b) -> pure $ pure b
     _                    -> pure Nothing
   fromValue v = fromValueMay v >>= \case
     Just b -> pure b
-    _      -> throwError $ Expectation @t @f @m TInt (Free v)
+    _      -> throwError $ Expectation @f @m TInt (Free v)
 
 instance Convertible e t f m
-  => FromValue Float m (NValue' t f m (NValue t f m)) where
+  => FromValue Float m (NValue' f m (Free (NValue' f m) t)) where
   fromValueMay = \case
     NVConstant' (NFloat b) -> pure $ pure b
     NVConstant' (NInt   i) -> pure $ pure (fromInteger i)
     _                      -> pure Nothing
   fromValue v = fromValueMay v >>= \case
     Just b -> pure b
-    _      -> throwError $ Expectation @t @f @m TFloat (Free v)
+    _      -> throwError $ Expectation @f @m TFloat (Free v)
 
 instance ( Convertible e t f m
-         , MonadValue (NValue t f m) m
-         , MonadEffects t f m
+         , MonadValue (NValue f m) m
+         , MonadEffects f m
          )
-  => FromValue NixString m (NValue' t f m (NValue t f m)) where
+  => FromValue NixString m (NValue' f m (Free (NValue' f m) t)) where
   fromValueMay = \case
     NVStr' ns -> pure $ pure ns
     NVPath' p ->
@@ -160,24 +160,24 @@ instance ( Convertible e t f m
     _ -> pure mempty
   fromValue v = fromValueMay v >>= \case
     Just b -> pure b
-    _      -> throwError $ Expectation @t @f @m (TString NoContext) (Free v)
+    _      -> throwError $ Expectation @f @m (TString NoContext) (Free v)
 
 instance Convertible e t f m
-  => FromValue ByteString m (NValue' t f m (NValue t f m)) where
+  => FromValue ByteString m (NValue' f m (Free (NValue' f m) t)) where
   fromValueMay = \case
     NVStr' ns -> pure $ encodeUtf8 <$> getStringNoContext  ns
     _         -> pure mempty
   fromValue v = fromValueMay v >>= \case
     Just b -> pure b
-    _      -> throwError $ Expectation @t @f @m (TString NoContext) (Free v)
+    _      -> throwError $ Expectation @f @m (TString NoContext) (Free v)
 
 newtype Path = Path { getPath :: FilePath }
     deriving Show
 
 instance ( Convertible e t f m
-         , MonadValue (NValue t f m) m
+         , MonadValue (NValue f m) m
          )
-  => FromValue Path m (NValue' t f m (NValue t f m)) where
+  => FromValue Path m (NValue' f m (Free (NValue' f m) t)) where
   fromValueMay = \case
     NVPath' p  -> pure $ pure (Path p)
     NVStr'  ns -> pure $ Path . Text.unpack <$> getStringNoContext  ns
@@ -187,75 +187,75 @@ instance ( Convertible e t f m
     _ -> pure Nothing
   fromValue v = fromValueMay v >>= \case
     Just b -> pure b
-    _      -> throwError $ Expectation @t @f @m TPath (Free v)
+    _      -> throwError $ Expectation @f @m TPath (Free v)
 
 instance Convertible e t f m
-  => FromValue [NValue t f m] m (NValue' t f m (NValue t f m)) where
+  => FromValue [Free (NValue' f m) t] m (NValue' f m (Free (NValue' f m) t)) where
   fromValueMay = \case
     NVList' l -> pure $ pure l
     _         -> pure mempty
   fromValue v = fromValueMay v >>= \case
     Just b -> pure b
-    _      -> throwError $ Expectation @t @f @m TList (Free v)
+    _      -> throwError $ Expectation @f @m TList (Free v)
 
 instance ( Convertible e t f m
-         , FromValue a m (NValue t f m)
+         , FromValue a m (NValue f m)
          )
-  => FromValue [a] m (Deeper (NValue' t f m (NValue t f m))) where
+  => FromValue [a] m (Deeper (NValue' f m (Free (NValue' f m) t))) where
   fromValueMay = \case
     Deeper (NVList' l) -> sequence <$> traverse fromValueMay l
     _                  -> pure mempty
   fromValue v = fromValueMay v >>= \case
     Just b -> pure b
-    _      -> throwError $ Expectation @t @f @m TList (Free (getDeeper v))
+    _      -> throwError $ Expectation @f @m TList (Free (getDeeper v))
 
 instance Convertible e t f m
-  => FromValue (AttrSet (NValue t f m)) m (NValue' t f m (NValue t f m)) where
+  => FromValue (AttrSet (Free (NValue' f m) t)) m (NValue' f m (Free (NValue' f m) t)) where
   fromValueMay = \case
     NVSet' s _ -> pure $ pure s
     _          -> pure mempty
   fromValue v = fromValueMay v >>= \case
     Just b -> pure b
-    _      -> throwError $ Expectation @t @f @m TSet (Free v)
+    _      -> throwError $ Expectation @f @m TSet (Free v)
 
 instance ( Convertible e t f m
-         , FromValue a m (NValue t f m)
+         , FromValue a m (NValue f m)
          )
-  => FromValue (AttrSet a) m (Deeper (NValue' t f m (NValue t f m))) where
+  => FromValue (AttrSet a) m (Deeper (NValue' f m (Free (NValue' f m) t))) where
   fromValueMay = \case
     Deeper (NVSet' s _) -> sequence <$> traverse fromValueMay s
     _                   -> pure mempty
   fromValue v = fromValueMay v >>= \case
     Just b -> pure b
-    _      -> throwError $ Expectation @t @f @m TSet (Free (getDeeper v))
+    _      -> throwError $ Expectation @f @m TSet (Free (getDeeper v))
 
 instance Convertible e t f m
-  => FromValue (AttrSet (NValue t f m), AttrSet SourcePos) m
-              (NValue' t f m (NValue t f m)) where
+  => FromValue (AttrSet (Free (NValue' f m) t), AttrSet SourcePos) m
+              (NValue' f m (Free (NValue' f m) t)) where
   fromValueMay = \case
     NVSet' s p -> pure $ pure (s, p)
     _          -> pure mempty
   fromValue v = fromValueMay v >>= \case
     Just b -> pure b
-    _      -> throwError $ Expectation @t @f @m TSet (Free v)
+    _      -> throwError $ Expectation @f @m TSet (Free v)
 
 instance ( Convertible e t f m
-         , FromValue a m (NValue t f m)
+         , FromValue a m (NValue f m)
          )
   => FromValue (AttrSet a, AttrSet SourcePos) m
-              (Deeper (NValue' t f m (NValue t f m))) where
+              (Deeper (NValue' f m (Free (NValue' f m) t))) where
   fromValueMay = \case
     Deeper (NVSet' s p) -> fmap (, p) . sequence <$> traverse fromValueMay s
     _                   -> pure mempty
   fromValue v = fromValueMay v >>= \case
     Just b -> pure b
-    _      -> throwError $ Expectation @t @f @m TSet (Free (getDeeper v))
+    _      -> throwError $ Expectation @f @m TSet (Free (getDeeper v))
 
 -- This instance needs IncoherentInstances, and only because of ToBuiltin
 instance ( Convertible e t f m
-         , FromValue a m (NValue' t f m (NValue t f m))
+         , FromValue a m (NValue' f m (NValue f m))
          )
-  => FromValue a m (Deeper (NValue' t f m (NValue t f m))) where
+  => FromValue a m (Deeper (NValue' f m (Free (NValue' f m) t))) where
   fromValueMay = fromValueMay . getDeeper
   fromValue    = fromValue . getDeeper
 
@@ -266,55 +266,55 @@ instance ( Convertible e t f m
 class ToValue a m v where
     toValue :: a -> m v
 
-instance (Convertible e t f m, ToValue a m (NValue' t f m (NValue t f m)))
-  => ToValue a m (NValue t f m) where
+instance (Convertible e t f m, ToValue a m (NValue' f m (NValue f m)))
+  => ToValue a m (Free (NValue' f m) t) where
   toValue = fmap Free . toValue
 
 instance ( Convertible e t f m
-         , ToValue a m (Deeper (NValue' t f m (NValue t f m)))
+         , ToValue a m (Deeper (NValue' f m (NValue f m)))
          )
-  => ToValue a m (Deeper (NValue t f m)) where
+  => ToValue a m (Deeper (Free (NValue' f m) t)) where
   toValue = fmap (fmap Free) . toValue
 
 instance Convertible e t f m
-  => ToValue () m (NValue' t f m (NValue t f m)) where
+  => ToValue () m (NValue' f m (Free (NValue' f m) t)) where
   toValue _ = pure . nvConstant' $ NNull
 
 instance Convertible e t f m
-  => ToValue Bool m (NValue' t f m (NValue t f m)) where
+  => ToValue Bool m (NValue' f m (Free (NValue' f m) t)) where
   toValue = pure . nvConstant' . NBool
 
 instance Convertible e t f m
-  => ToValue Int m (NValue' t f m (NValue t f m)) where
+  => ToValue Int m (NValue' f m (Free (NValue' f m) t)) where
   toValue = pure . nvConstant' . NInt . toInteger
 
 instance Convertible e t f m
-  => ToValue Integer m (NValue' t f m (NValue t f m)) where
+  => ToValue Integer m (NValue' f m (Free (NValue' f m) t)) where
   toValue = pure . nvConstant' . NInt
 
 instance Convertible e t f m
-  => ToValue Float m (NValue' t f m (NValue t f m)) where
+  => ToValue Float m (NValue' f m (Free (NValue' f m) t)) where
   toValue = pure . nvConstant' . NFloat
 
 instance Convertible e t f m
-  => ToValue NixString m (NValue' t f m (NValue t f m)) where
+  => ToValue NixString m (NValue' f m (Free (NValue' f m) t)) where
   toValue = pure . nvStr'
 
 instance Convertible e t f m
-  => ToValue ByteString m (NValue' t f m (NValue t f m)) where
+  => ToValue ByteString m (NValue' f m (Free (NValue' f m) t)) where
   toValue = pure . nvStr' . makeNixStringWithoutContext . decodeUtf8
 
 instance Convertible e t f m
-  => ToValue Path m (NValue' t f m (NValue t f m)) where
+  => ToValue Path m (NValue' f m (Free (NValue' f m) t)) where
   toValue = pure . nvPath' . getPath
 
 instance Convertible e t f m
-  => ToValue StorePath m (NValue' t f m (NValue t f m)) where
+  => ToValue StorePath m (NValue' f m (Free (NValue' f m) t)) where
   toValue = toValue . Path . unStorePath
 
 instance ( Convertible e t f m
          )
-  => ToValue SourcePos m (NValue' t f m (NValue t f m)) where
+  => ToValue SourcePos m (NValue' f m (Free (NValue' f m) t)) where
   toValue (SourcePos f l c) = do
     f' <- toValue (makeNixStringWithoutContext (Text.pack f))
     l' <- toValue (unPos l)
@@ -324,33 +324,33 @@ instance ( Convertible e t f m
 
 -- | With 'ToValue', we can always act recursively
 instance Convertible e t f m
-  => ToValue [NValue t f m] m (NValue' t f m (NValue t f m)) where
+  => ToValue [Free (NValue' f m) t] m (NValue' f m (Free (NValue' f m) t)) where
   toValue = pure . nvList'
 
-instance (Convertible e t f m, ToValue a m (NValue t f m))
-  => ToValue [a] m (Deeper (NValue' t f m (NValue t f m))) where
+instance (Convertible e t f m, ToValue a m (NValue f m))
+  => ToValue [a] m (Deeper (NValue' f m (Free (NValue' f m) t))) where
   toValue = fmap (Deeper . nvList') . traverse toValue
 
 instance Convertible e t f m
-  => ToValue (AttrSet (NValue t f m)) m (NValue' t f m (NValue t f m)) where
+  => ToValue (AttrSet (Free (NValue' f m) t)) m (NValue' f m (Free (NValue' f m) t)) where
   toValue s = pure $ nvSet' s mempty
 
-instance (Convertible e t f m, ToValue a m (NValue t f m))
-  => ToValue (AttrSet a) m (Deeper (NValue' t f m (NValue t f m))) where
+instance (Convertible e t f m, ToValue a m (NValue f m))
+  => ToValue (AttrSet a) m (Deeper (NValue' f m (Free (NValue' f m) t))) where
   toValue s = (Deeper .) . nvSet' <$> traverse toValue s <*> pure mempty
 
 instance Convertible e t f m
-  => ToValue (AttrSet (NValue t f m), AttrSet SourcePos) m
-            (NValue' t f m (NValue t f m)) where
+  => ToValue (AttrSet (Free (NValue' f m) t), AttrSet SourcePos) m
+            (NValue' f m (Free (NValue' f m) t)) where
   toValue (s, p) = pure $ nvSet' s p
 
-instance (Convertible e t f m, ToValue a m (NValue t f m))
+instance (Convertible e t f m, ToValue a m (NValue f m))
   => ToValue (AttrSet a, AttrSet SourcePos) m
-            (Deeper (NValue' t f m (NValue t f m))) where
+            (Deeper (NValue' f m (Free (NValue' f m) t))) where
   toValue (s, p) = (Deeper .) . nvSet' <$> traverse toValue s <*> pure p
 
 instance Convertible e t f m
-  => ToValue NixLikeContextValue m (NValue' t f m (NValue t f m)) where
+  => ToValue NixLikeContextValue m (NValue' f m (Free (NValue' f m) t)) where
   toValue nlcv = do
     path <- if nlcvPath nlcv then pure <$> toValue True else pure Nothing
     allOutputs <- if nlcvAllOutputs nlcv
@@ -359,7 +359,7 @@ instance Convertible e t f m
     outputs <- do
       let outputs =
             makeNixStringWithoutContext <$> nlcvOutputs nlcv
-      ts :: [NValue t f m] <- traverse toValue outputs
+      ts :: [NValue f m] <- traverse toValue outputs
       case ts of
         [] -> pure Nothing
         _  -> pure <$> toValue ts
@@ -369,8 +369,8 @@ instance Convertible e t f m
       , ("outputs",) <$> outputs
       ]
 
-instance Convertible e t f m => ToValue () m (NExprF (NValue t f m)) where
+instance Convertible e t f m => ToValue () m (NExprF (Free (NValue' f m) t)) where
   toValue _ = pure . NConstant $ NNull
 
-instance Convertible e t f m => ToValue Bool m (NExprF (NValue t f m)) where
+instance Convertible e t f m => ToValue Bool m (NExprF (Free (NValue' f m) t)) where
   toValue = pure . NConstant . NBool
diff --git a/src/Nix/Effects.hs b/src/Nix/Effects.hs
index 86828c842..2e68d9ed9 100644
--- a/src/Nix/Effects.hs
+++ b/src/Nix/Effects.hs
@@ -24,6 +24,8 @@ import           Prelude                 hiding ( putStr
                                                 )
 import qualified Prelude
 
+import           Control.Monad.Trans.Reader
+import           Control.Monad.Trans.State
 import           Control.Monad.Trans
 import qualified Data.HashSet                  as HS
 import           Data.Text                      ( Text )
@@ -37,6 +39,7 @@ import           Nix.Expr
 import           Nix.Frames              hiding ( Proxy )
 import           Nix.Parser
 import           Nix.Render
+import           Nix.Scope.Basic
 import           Nix.Utils
 import           Nix.Value
 import qualified Paths_hnix
@@ -61,25 +64,22 @@ class (MonadFile m,
        MonadPaths m,
        MonadInstantiate m,
        MonadExec m,
-       MonadIntrospect m) => MonadEffects t f m where
+       MonadIntrospect m) => MonadEffects f m where
   -- | Determine the absolute path of relative path in the current context
   makeAbsolutePath :: FilePath -> m FilePath
   findEnvPath :: String -> m FilePath
 
   -- | Having an explicit list of sets corresponding to the NIX_PATH
   -- and a file path try to find an existing path
-  findPath :: [NValue t f m] -> FilePath -> m FilePath
+  findPath :: [NValue f m] -> FilePath -> m FilePath
 
-  importPath :: FilePath -> m (NValue t f m)
+  importPath :: FilePath -> m (NValue f m)
   pathToDefaultNix :: FilePath -> m FilePath
 
-  derivationStrict :: NValue t f m -> m (NValue t f m)
+  derivationStrict :: NValue f m -> m (NValue f m)
 
   traceEffect :: String -> m ()
 
-instance (MonadFix1T t m, MonadStore m) => MonadStore (Fix1T t m) where
-  addToStore a b c d = lift $ addToStore a b c d
-  addTextToStore' a b c d = lift $ addTextToStore' a b c d
 
 class Monad m => MonadIntrospect m where
   recursiveSize :: a -> m Word
@@ -298,7 +298,11 @@ addPath p = either throwError pure =<< addToStore (T.pack $ takeFileName p) p Tr
 toFile_ :: (Framed e m, MonadStore m) => FilePath -> String -> m StorePath
 toFile_ p contents = addTextToStore (T.pack p) (T.pack contents) HS.empty False
 
--- All of the following type classes defer to the underlying 'm'.
+-- | All of the following type classes defer to the underlying 'm'.
+
+instance MonadStore m => MonadStore (ReaderT r m)
+deriving instance MonadStore m => MonadStore (ScopeT binding r m)
+instance MonadStore m => MonadStore (StateT s m)
 
 deriving instance MonadPutStr (t (Fix1 t)) => MonadPutStr (Fix1 t)
 deriving instance MonadHttp (t (Fix1 t)) => MonadHttp (Fix1 t)
@@ -308,10 +312,11 @@ deriving instance MonadInstantiate (t (Fix1 t)) => MonadInstantiate (Fix1 t)
 deriving instance MonadExec (t (Fix1 t)) => MonadExec (Fix1 t)
 deriving instance MonadIntrospect (t (Fix1 t)) => MonadIntrospect (Fix1 t)
 
-deriving instance MonadPutStr (t (Fix1T t m) m) => MonadPutStr (Fix1T t m)
-deriving instance MonadHttp (t (Fix1T t m) m) => MonadHttp (Fix1T t m)
 deriving instance MonadEnv (t (Fix1T t m) m) => MonadEnv (Fix1T t m)
-deriving instance MonadPaths (t (Fix1T t m) m) => MonadPaths (Fix1T t m)
-deriving instance MonadInstantiate (t (Fix1T t m) m) => MonadInstantiate (Fix1T t m)
 deriving instance MonadExec (t (Fix1T t m) m) => MonadExec (Fix1T t m)
+deriving instance MonadHttp (t (Fix1T t m) m) => MonadHttp (Fix1T t m)
+deriving instance MonadInstantiate (t (Fix1T t m) m) => MonadInstantiate (Fix1T t m)
 deriving instance MonadIntrospect (t (Fix1T t m) m) => MonadIntrospect (Fix1T t m)
+deriving instance MonadPaths (t (Fix1T t m) m) => MonadPaths (Fix1T t m)
+deriving instance MonadPutStr (t (Fix1T t m) m) => MonadPutStr (Fix1T t m)
+deriving instance MonadStore (t (Fix1T t m) m) => MonadStore (Fix1T t m)
diff --git a/src/Nix/Effects/Basic.hs b/src/Nix/Effects/Basic.hs
index d8fc2369c..4629ac317 100644
--- a/src/Nix/Effects/Basic.hs
+++ b/src/Nix/Effects/Basic.hs
@@ -1,6 +1,7 @@
 {-# LANGUAGE ApplicativeDo #-}
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE GADTs #-}
 {-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE ScopedTypeVariables #-}
@@ -42,7 +43,7 @@ import           GHC.DataSize
 #endif
 #endif
 
-defaultMakeAbsolutePath :: MonadNix e t f m => FilePath -> m FilePath
+defaultMakeAbsolutePath :: MonadNix e f m => FilePath -> m FilePath
 defaultMakeAbsolutePath origPath = do
   origPathExpanded <- expandHomePath origPath
   absPath          <- if isAbsolute origPathExpanded
@@ -88,32 +89,32 @@ x <///> y | isAbsolute y || "." `isPrefixOf` y = x </> y
     joinPath $ head
       [ xs <> drop (length tx) ys | tx <- tails xs, tx `elem` inits ys ]
 
-defaultFindEnvPath :: MonadNix e t f m => String -> m FilePath
+defaultFindEnvPath :: MonadNix e f m => String -> m FilePath
 defaultFindEnvPath = findEnvPathM
 
-findEnvPathM :: forall e t f m . MonadNix e t f m => FilePath -> m FilePath
+findEnvPathM :: forall e f m . MonadNix e f m => FilePath -> m FilePath
 findEnvPathM name = do
   mres <- lookupVar "__nixPath"
   case mres of
     Nothing -> error "impossible"
-    Just x  -> demand x $ fromValue >=> \(l :: [NValue t f m]) ->
+    Just x  -> demand x $ fromValue >=> \(l :: [NValue f m]) ->
       findPathBy nixFilePath l name
  where
-  nixFilePath :: MonadEffects t f m => FilePath -> m (Maybe FilePath)
+  nixFilePath :: MonadEffects f m => FilePath -> m (Maybe FilePath)
   nixFilePath path = do
-    absPath <- makeAbsolutePath @t @f path
+    absPath <- makeAbsolutePath @f path
     isDir   <- doesDirectoryExist absPath
     absFile <- if isDir
-      then makeAbsolutePath @t @f $ absPath </> "default.nix"
+      then makeAbsolutePath @f $ absPath </> "default.nix"
       else pure absPath
     exists <- doesFileExist absFile
     pure $ if exists then pure absFile else mempty
 
 findPathBy
-  :: forall e t f m
-   . MonadNix e t f m
+  :: forall e f m
+   . MonadNix e f m
   => (FilePath -> m (Maybe FilePath))
-  -> [NValue t f m]
+  -> [NValue f m]
   -> FilePath
   -> m FilePath
 findPathBy finder ls name = do
@@ -128,10 +129,10 @@ findPathBy finder ls name = do
         <> " (add it's using $NIX_PATH or -I)"
     Just path -> pure path
  where
-  go :: Maybe FilePath -> NValue t f m -> m (Maybe FilePath)
+  go :: Maybe FilePath -> NValue f m -> m (Maybe FilePath)
   go p@(Just _) _ = pure p
   go Nothing l =
-    demand l $ fromValue >=> \(s :: HashMap Text (NValue t f m)) -> do
+    demand l $ fromValue >=> \(s :: HashMap Text (NValue f m)) -> do
       p <- resolvePath s
       demand p $ fromValue >=> \(Path path) -> case M.lookup "prefix" s of
         Nothing -> tryPath path mempty
@@ -159,7 +160,7 @@ findPathBy finder ls name = do
           <> show s
 
 fetchTarball
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
+  :: forall e f m . MonadNix e f m => NValue f m -> m (NValue f m)
 fetchTarball = flip demand $ \case
   NVSet s _ -> case M.lookup "url" s of
     Nothing ->
@@ -172,7 +173,7 @@ fetchTarball = flip demand $ \case
       $  "builtins.fetchTarball: Expected URI or set, got "
       <> show v
  where
-  go :: Maybe (NValue t f m) -> NValue t f m -> m (NValue t f m)
+  go :: Maybe (NValue f m) -> NValue f m -> m (NValue f m)
   go msha = \case
     NVStr ns -> fetch (stringIgnoreContext ns) msha
     v ->
@@ -182,7 +183,7 @@ fetchTarball = flip demand $ \case
         <> show v
 
 {- jww (2018-04-11): This should be written using pipes in another module
-    fetch :: Text -> Maybe (NThunk m) -> m (NValue t f m)
+    fetch :: Text -> Maybe (NThunk m) -> m (NValue f m)
     fetch uri msha = case takeExtension (Text.unpack uri) of
         ".tgz" -> undefined
         ".gz"  -> undefined
@@ -193,7 +194,7 @@ fetchTarball = flip demand $ \case
                   <> ext <> "'"
 -}
 
-  fetch :: Text -> Maybe (NValue t f m) -> m (NValue t f m)
+  fetch :: Text -> Maybe (NValue f m) -> m (NValue f m)
   fetch uri Nothing =
     nixInstantiateExpr $ "builtins.fetchTarball \"" <> Text.unpack uri <> "\""
   fetch url (Just t) = demand t $ fromValue >=> \nsSha ->
@@ -207,27 +208,27 @@ fetchTarball = flip demand $ \case
           <> Text.unpack sha
           <> "\"; }"
 
-defaultFindPath :: MonadNix e t f m => [NValue t f m] -> FilePath -> m FilePath
+defaultFindPath :: MonadNix e f m => [NValue f m] -> FilePath -> m FilePath
 defaultFindPath = findPathM
 
 findPathM
-  :: forall e t f m
-   . MonadNix e t f m
-  => [NValue t f m]
+  :: forall e f m
+   . MonadNix e f m
+  => [NValue f m]
   -> FilePath
   -> m FilePath
 findPathM = findPathBy existingPath
  where
-  existingPath :: MonadEffects t f m => FilePath -> m (Maybe FilePath)
+  existingPath :: MonadEffects f m => FilePath -> m (Maybe FilePath)
   existingPath path = do
-    apath  <- makeAbsolutePath @t @f path
+    apath  <- makeAbsolutePath @f path
     exists <- doesPathExist apath
     pure $ if exists then pure apath else mempty
 
 defaultImportPath
-  :: (MonadNix e t f m, MonadState (HashMap FilePath NExprLoc, b) m)
+  :: (MonadNix e f m, MonadState (HashMap FilePath NExprLoc, b) m)
   => FilePath
-  -> m (NValue t f m)
+  -> m (NValue f m)
 defaultImportPath path = do
   traceM $ "Importing file " <> path
   withFrame Info (ErrorCall $ "While importing file " <> show path) $ do
@@ -245,7 +246,7 @@ defaultImportPath path = do
             modify (\(a, b) -> (M.insert path expr a, b))
             pure expr
 
-defaultPathToDefaultNix :: MonadNix e t f m => FilePath -> m FilePath
+defaultPathToDefaultNix :: MonadNix e f m => FilePath -> m FilePath
 defaultPathToDefaultNix = pathToDefaultNixFile
 
 -- Given a path, determine the nix file to load
diff --git a/src/Nix/Effects/Derivation.hs b/src/Nix/Effects/Derivation.hs
index 4398140fe..fb733f669 100644
--- a/src/Nix/Effects/Derivation.hs
+++ b/src/Nix/Effects/Derivation.hs
@@ -5,9 +5,9 @@
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE RecordWildCards #-}
 {-# LANGUAGE ScopedTypeVariables #-}
-{-# LANGUAGE TupleSections #-}
 {-# LANGUAGE TypeApplications #-}
- 
+{-# LANGUAGE TypeFamilies #-}
+
 
 module Nix.Effects.Derivation ( defaultDerivationStrict ) where
 
@@ -15,6 +15,7 @@ import           Prelude                 hiding ( readFile )
 
 import           Control.Arrow                  ( first, second )
 import           Control.Monad                  ( (>=>), forM, when )
+import           Control.Monad.Catch
 import           Control.Monad.Writer           ( join, lift )
 import           Control.Monad.State            ( MonadState, gets, modify )
 
@@ -102,7 +103,7 @@ writeDerivation drv@Derivation{inputs, name} = do
 
 -- | Traverse the graph of inputDrvs to replace fixed output derivations with their fixed output hash.
 -- this avoids propagating changes to their .drv when the output hash stays the same.
-hashDerivationModulo :: (MonadNix e t f m, MonadState (b, MS.HashMap Text Text) m) => Derivation -> m (Store.Digest 'Store.SHA256)
+hashDerivationModulo :: (MonadNix e f m, MonadState (b, MS.HashMap Text Text) m) => Derivation -> m (Store.Digest 'Store.SHA256)
 hashDerivationModulo (Derivation {
     mFixed = Just (Store.SomeDigest (digest :: Store.Digest hashType)),
     outputs,
@@ -226,8 +227,8 @@ derivationParser = do
     _ -> (Nothing, Flat)
 
 
-defaultDerivationStrict :: forall e t f m b. (MonadNix e t f m, MonadState (b, MS.HashMap Text Text) m) => NValue t f m -> m (NValue t f m)
-defaultDerivationStrict = fromValue @(AttrSet (NValue t f m)) >=> \s -> do
+defaultDerivationStrict :: forall e f m b. (MonadNix e f m, MonadState (b, MS.HashMap Text Text) m) => NValue f m -> m (NValue f m)
+defaultDerivationStrict = fromValue @(AttrSet (NValue f m)) >=> \s -> do
     (drv, ctx) <- runWithStringContextT' $ buildDerivationWithContext s
     drvName <- makeStorePathName $ name drv
     let inputs = toStorePaths ctx
@@ -288,10 +289,10 @@ defaultDerivationStrict = fromValue @(AttrSet (NValue t f m)) >=> \s -> do
 -- | Build a derivation in a context collecting string contexts.
 -- This is complex from a typing standpoint, but it allows to perform the
 -- full computation without worrying too much about all the string's contexts.
-buildDerivationWithContext :: forall e t f m. (MonadNix e t f m) => AttrSet (NValue t f m) -> WithStringContextT m Derivation
+buildDerivationWithContext :: forall e f m. (MonadNix e f m) => AttrSet (NValue f m) -> WithStringContextT m Derivation
 buildDerivationWithContext drvAttrs = do
     -- Parse name first, so we can add an informative frame
-    drvName     <- getAttr   "name"                      $ extractNixString >=> assertDrvStoreName
+    drvName       <- getAttr  "name"                       $ extractNixString >=> assertDrvStoreName
     withFrame' Info (ErrorCall $ "While evaluating derivation " <> show drvName) $ do
 
       useJson     <- getAttrOr "__structuredAttrs" False   $ pure
@@ -338,10 +339,10 @@ buildDerivationWithContext drvAttrs = do
   where
     -- common functions, lifted to WithStringContextT
 
-    demand' :: NValue t f m -> (NValue t f m -> WithStringContextT m a) -> WithStringContextT m a
+    demand' :: NValue f m -> (NValue f m -> WithStringContextT m a) -> WithStringContextT m a
     demand' v f = join $ lift $ demand v (pure . f)
 
-    fromValue' :: (FromValue a m (NValue' t f m (NValue t f m)), MonadNix e t f m) => NValue t f m -> WithStringContextT m a
+    fromValue' :: (FromValue a m (NValue' f m (NValue f m)), MonadNix e f m) => NValue f m -> WithStringContextT m a
     fromValue' = lift . fromValue
 
     withFrame' :: (Framed e m, Exception s) => NixLevel -> s -> WithStringContextT m a -> WithStringContextT m a
@@ -349,20 +350,22 @@ buildDerivationWithContext drvAttrs = do
 
     -- shortcuts to get the (forced) value of an AttrSet field
 
-    getAttrOr' :: forall v a. (MonadNix e t f m, FromValue v m (NValue' t f m (NValue t f m)))
+    getAttrOr' :: forall v a. (MonadNix e f m, FromValue v m (NValue' f m (NValue f m)))
       => Text -> m a -> (v -> WithStringContextT m a) -> WithStringContextT m a
     getAttrOr' n d f = case M.lookup n drvAttrs of
       Nothing -> lift d
       Just v  -> withFrame' Info (ErrorCall $ "While evaluating attribute '" <> show n <> "'") $
                    fromValue' v >>= f
 
+    getAttrOr :: forall v a. (MonadNix e f m, FromValue v m (NValue' f m (NValue f m)))
+      => Text -> a -> (v -> WithStringContextT m a) -> WithStringContextT m a
     getAttrOr n d f = getAttrOr' n (pure d) f
 
     getAttr n = getAttrOr' n (throwError $ ErrorCall $ "Required attribute '" <> show n <> "' not found.")
 
     -- Test validity for fields
 
-    assertDrvStoreName :: MonadNix e t f m => Text -> WithStringContextT m Text
+    assertDrvStoreName :: MonadNix e f m => Text -> WithStringContextT m Text
     assertDrvStoreName name = lift $ do
       let invalid c = not $ isAscii c && (isAlphaNum c || c `elem` ("+-._?=" :: String)) -- isAlphaNum allows non-ascii chars.
       let failWith reason = throwError $ ErrorCall $ "Store name " <> show name <> " " <> reason
@@ -372,17 +375,17 @@ buildDerivationWithContext drvAttrs = do
       when (".drv" `Text.isSuffixOf` name) $ failWith "is not allowed to end in '.drv'"
       pure name
 
-    extractNoCtx :: MonadNix e t f m => NixString -> WithStringContextT m Text
+    extractNoCtx :: MonadNix e f m => NixString -> WithStringContextT m Text
     extractNoCtx ns = case getStringNoContext ns of
       Nothing -> lift $ throwError $ ErrorCall $ "The string " <> show ns <> " is not allowed to have a context."
       Just v -> pure v
 
-    assertNonNull :: MonadNix e t f m => Text -> WithStringContextT m Text
+    assertNonNull :: MonadNix e f m => Text -> WithStringContextT m Text
     assertNonNull t = do
       when (Text.null t) $ lift $ throwError $ ErrorCall "Value must not be empty"
       pure t
 
-    parseHashMode :: MonadNix e t f m => Text -> WithStringContextT m HashMode
+    parseHashMode :: MonadNix e f m => Text -> WithStringContextT m HashMode
     parseHashMode = \case
       "flat" ->      pure Flat
       "recursive" -> pure Recursive
diff --git a/src/Nix/Eval.hs b/src/Nix/Eval.hs
index e790ce226..891c4aa7e 100644
--- a/src/Nix/Eval.hs
+++ b/src/Nix/Eval.hs
@@ -8,6 +8,7 @@
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TupleSections #-}
 {-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE RecursiveDo #-}
 
 
 
@@ -80,7 +81,7 @@ class (Show v, Monad m) => MonadEval v m where
 
 type MonadNixEval v m
   = ( MonadEval v m
-  , Scoped v m
+  , Scoped m v m
   , MonadValue v m
   , MonadFix m
   , ToValue Bool m v
@@ -126,8 +127,8 @@ eval (NEnvPath     p      ) = evalEnvPath p
 eval (NUnary op arg       ) = evalUnary op =<< arg
 
 eval (NBinary NApp fun arg) = do
-  scope <- currentScopes :: m (Scopes m v)
-  fun >>= (`evalApp` withScopes scope arg)
+  argD <- defer arg
+  fun >>= (`evalApp` pure argD)
 
 eval (NBinary op   larg rarg) = larg >>= evalBinary op ?? rarg
 
@@ -137,8 +138,7 @@ eval (NSelect aset attr alt ) = evalSelect aset attr >>= either go id
 eval (NHasAttr aset attr) = evalSelect aset attr >>= toValue . isRight
 
 eval (NList l           ) = do
-  scope <- currentScopes
-  for l (defer @v @m . withScopes @v scope) >>= toValue
+  for l defer >>= toValue
 
 eval (NSet NNonRecursive binds) =
   evalBinds False (desugarBinds (eval . NSet NNonRecursive) binds) >>= toValue
@@ -174,8 +174,7 @@ evalWithAttrSet aset body = do
   -- each time a name is looked up within the weak scope, and we want to be
   -- sure the action it evaluates is to force a thunk, so its value is only
   -- computed once.
-  scope <- currentScopes :: m (Scopes m v)
-  s     <- defer $ withScopes scope aset
+  s     <- defer aset
   let s' = demand s $ fmap fst . fromValue @(AttrSet v, AttrSet SourcePos)
   pushWeakScope s' body
 
@@ -244,8 +243,8 @@ evalBinds
   -> [Binding (m v)]
   -> m (AttrSet v, AttrSet SourcePos)
 evalBinds recursive binds = do
-  scope <- currentScopes :: m (Scopes m v)
-  buildResult scope . concat =<< mapM (go scope) (moveOverridesLast binds)
+  rec result <- buildResult . concat =<< mapM (go $ fst result) (moveOverridesLast binds)
+  pure result
  where
   moveOverridesLast = uncurry (<>) . partition
     (\case
@@ -253,7 +252,7 @@ evalBinds recursive binds = do
       _ -> True
     )
 
-  go :: Scopes m v -> Binding (m v) -> m [([Text], SourcePos, m v)]
+  go :: AttrSet v -> Binding (m v) -> m [([Text], SourcePos, m v)]
   go _ (NamedVar (StaticKey "__overrides" :| []) finalValue pos) =
     finalValue >>= fromValue >>= \(o', p') ->
           -- jww (2018-05-09): What to do with the key position here?
@@ -284,35 +283,36 @@ evalBinds recursive binds = do
       ([], _, _) -> mempty
       result     -> [result]
 
-  go scope (Inherit ms names pos) =
+  go scope (Inherit ms names pos) = do
     fmap catMaybes $ forM names $ evalSetterKeyName >=> \case
       Nothing  -> pure Nothing
-      Just key -> pure $ Just
-        ( [key]
-        , pos
-        , do
+      Just key -> do
+        x <- defer $ do
           mv <- case ms of
-            Nothing -> withScopes scope $ lookupVar key
+            Nothing -> lookupVar key
             Just s ->
-              s >>= fromValue @(AttrSet v, AttrSet SourcePos) >>= \(attrset, _) ->
-                clearScopes @v $ pushScope attrset $ lookupVar key
+              -- The inherit source expression is evaluated in the recursive context
+              -- if this is a recursive record
+              pushScope scope s >>= fromValue @(AttrSet v, AttrSet SourcePos) >>= \(s, _) ->
+                lookupVarScopes key (Scopes [Scope s] mempty)
           case mv of
-            Nothing -> attrMissing (key :| []) Nothing
+            Nothing -> attrMissing (key :| mempty) Nothing
             Just v  -> demand v pure
-        )
+        pure $ Just
+          ( [key]
+          , pos
+          , pure x
+          )
 
   buildResult
-    :: Scopes m v
-    -> [([Text], SourcePos, m v)]
+    :: [([Text], SourcePos, m v)]
     -> m (AttrSet v, AttrSet SourcePos)
-  buildResult scope bindings = do
+  buildResult bindings = do
     (s, p) <- foldM insert (M.empty, M.empty) bindings
-    res <- if recursive then loebM (encapsulate <$> s) else traverse mkThunk s
-    pure (res, p)
+    res <- if recursive then loebM (encapsulate <$> s) else traverse defer s
+    return (res, p)
    where
-    mkThunk = defer . withScopes scope
-
-    encapsulate f attrs = mkThunk . pushScope attrs $ f
+    encapsulate f attrs = defer . pushScope attrs $ f
 
     insert (m, p) (path, pos, value) = attrSetAlter path pos m p value
 
@@ -378,27 +378,26 @@ assembleString = \case
 buildArgument
   :: forall v m . MonadNixEval v m => Params (m v) -> m v -> m (AttrSet v)
 buildArgument params arg = do
-  scope <- currentScopes :: m (Scopes m v)
+  argD <- defer arg
   case params of
-    Param name -> M.singleton name <$> defer (withScopes scope arg)
+    Param name -> pure $ M.singleton name argD
     ParamSet s isVariadic m ->
       arg >>= fromValue @(AttrSet v, AttrSet SourcePos) >>= \(args, _) -> do
         let inject = case m of
               Nothing -> id
-              Just n  -> M.insert n $ const $ defer (withScopes scope arg)
+              Just n  -> M.insert n $ const $ pure argD
         loebM
-          (inject $ M.mapMaybe id $ ialignWith (assemble scope isVariadic)
+          (inject $ M.mapMaybe id $ ialignWith (assemble isVariadic)
                                                args
                                                (M.fromList s)
           )
  where
   assemble
-    :: Scopes m v
-    -> Bool
+    :: Bool
     -> Text
     -> These v (Maybe (m v))
     -> Maybe (AttrSet v -> m v)
-  assemble scope isVariadic k = \case
+  assemble isVariadic k = \case
     That Nothing ->
       pure
         $  const
@@ -407,7 +406,7 @@ buildArgument params arg = do
         $  "Missing value for parameter: "
         <> show k
     That (Just f) ->
-      pure $ \args -> defer $ withScopes scope $ pushScope args f
+      pure $ \args -> defer $ pushScope args f
     This _
       | isVariadic
       -> Nothing
@@ -427,7 +426,7 @@ addSourcePositions f v@(Fix (Compose (Ann ann _))) =
 
 addStackFrames
   :: forall v e m a
-   . (Scoped v m, Framed e m, Typeable v, Typeable m)
+   . (Scoped m v m, Framed e m, Typeable v, Typeable m)
   => Transform NExprLocF (m a)
 addStackFrames f v = do
   scopes <- currentScopes :: m (Scopes m v)
diff --git a/src/Nix/Exec.hs b/src/Nix/Exec.hs
index 3329c1c9e..9881e547e 100644
--- a/src/Nix/Exec.hs
+++ b/src/Nix/Exec.hs
@@ -10,6 +10,7 @@
 {-# LANGUAGE PartialTypeSignatures #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE TupleSections #-}
 {-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE TypeFamilies #-}
@@ -27,9 +28,11 @@ import           Prelude                 hiding ( putStr
                                                 )
 
 import           Control.Applicative
+import           Control.Comonad
 import           Control.Monad
 import           Control.Monad.Catch     hiding ( catchJust )
 import           Control.Monad.Fix
+import           Control.Monad.Free
 import           Control.Monad.Reader
 import           Data.Fix
 import qualified Data.HashMap.Lazy             as M
@@ -67,86 +70,88 @@ import           GHC.DataSize
 #endif
 #endif
 
-type MonadCited t f m
-  = ( HasCitations m (NValue t f m) t
-  , HasCitations1 m (NValue t f m) f
+type MonadCited f m
+  = ( HasCitations m (NValue f m) (Thunk m)
+  , HasCitations1 m (NValue f m) f
   , MonadDataContext f m
   )
 
 nvConstantP
-  :: MonadCited t f m => Provenance m (NValue t f m) -> NAtom -> NValue t f m
+  :: MonadCited f m => Provenance m (NValue f m) -> NAtom -> NValue f m
 nvConstantP p x = addProvenance p (nvConstant x)
 
 nvStrP
-  :: MonadCited t f m
-  => Provenance m (NValue t f m)
+  :: MonadCited f m
+  => Provenance m (NValue f m)
   -> NixString
-  -> NValue t f m
+  -> NValue f m
 nvStrP p ns = addProvenance p (nvStr ns)
 
 nvPathP
-  :: MonadCited t f m => Provenance m (NValue t f m) -> FilePath -> NValue t f m
+  :: MonadCited f m => Provenance m (NValue f m) -> FilePath -> NValue f m
 nvPathP p x = addProvenance p (nvPath x)
 
 nvListP
-  :: MonadCited t f m
-  => Provenance m (NValue t f m)
-  -> [NValue t f m]
-  -> NValue t f m
+  :: MonadCited f m
+  => Provenance m (NValue f m)
+  -> [NValue f m]
+  -> NValue f m
 nvListP p l = addProvenance p (nvList l)
 
 nvSetP
-  :: MonadCited t f m
-  => Provenance m (NValue t f m)
-  -> AttrSet (NValue t f m)
+  :: MonadCited f m
+  => Provenance m (NValue f m)
+  -> AttrSet (NValue f m)
   -> AttrSet SourcePos
-  -> NValue t f m
+  -> NValue f m
 nvSetP p s x = addProvenance p (nvSet s x)
 
 nvClosureP
-  :: MonadCited t f m
-  => Provenance m (NValue t f m)
+  :: MonadCited f m
+  => Provenance m (NValue f m)
   -> Params ()
-  -> (NValue t f m -> m (NValue t f m))
-  -> NValue t f m
+  -> (NValue f m -> m (NValue f m))
+  -> NValue f m
 nvClosureP p x f = addProvenance p (nvClosure x f)
 
 nvBuiltinP
-  :: MonadCited t f m
-  => Provenance m (NValue t f m)
+  :: MonadCited f m
+  => Provenance m (NValue f m)
   -> String
-  -> (NValue t f m -> m (NValue t f m))
-  -> NValue t f m
+  -> (NValue f m -> m (NValue f m))
+  -> NValue f m
 nvBuiltinP p name f = addProvenance p (nvBuiltin name f)
 
 type MonadCitedThunks t f m
-  = ( MonadThunk t m (NValue t f m)
-  , MonadDataErrorContext t f m
-  , HasCitations m (NValue t f m) t
-  , HasCitations1 m (NValue t f m) f
+  = ( MonadThunk m, Thunk m ~ t, ThunkValue m ~ NValue f m
+  , MonadDataErrorContext f m
+  , HasCitations m (NValue f m) t
+  , HasCitations1 m (NValue f m) f
   )
 
-type MonadNix e t f m
+type MonadNix e f m
   = ( Has e SrcSpan
   , Has e Options
-  , Scoped (NValue t f m) m
+  , Scoped m (NValue f m) m
   , Framed e m
   , MonadFix m
   , MonadCatch m
   , MonadThrow m
   , Alternative m
-  , MonadEffects t f m
-  , MonadCitedThunks t f m
-  , MonadValue (NValue t f m) m
+  , MonadEffects f m
+  , MonadCitedThunks (Thunk m) f m
+  , MonadValue (NValue f m) m
   )
 
-data ExecFrame t f m = Assertion SrcSpan (NValue t f m)
-    deriving (Show, Typeable)
+data ExecFrame f m = Assertion SrcSpan (NValue f m)
+    deriving (Typeable)
 
-instance MonadDataErrorContext t f m => Exception (ExecFrame t f m)
+deriving instance (Show (Thunk m), Comonad f) => Show (ExecFrame f m)
 
-nverr :: forall e t f s m a . (MonadNix e t f m, Exception s) => s -> m a
-nverr = evalError @(NValue t f m)
+instance MonadDataErrorContext f m => Exception (ExecFrame f m)
+
+nverr :: forall e f s m a . (MonadNix e f m, Exception s) => s -> m a
+nverr = evalError @(NValue f m)
 
 currentPos :: forall e m . (MonadReader e m, Has e SrcSpan) => m SrcSpan
 currentPos = asks (view hasLens)
@@ -154,11 +159,9 @@ currentPos = asks (view hasLens)
 wrapExprLoc :: SrcSpan -> NExprLocF r -> NExprLoc
 wrapExprLoc span x = Fix (Fix (NSym_ span "<?>") <$ x)
 
---  2021-01-07: NOTE: This instance belongs to be beside MonadEval type class.
--- Currently instance is stuck in orphanage between the requirements to be MonadEval, aka Eval stage, and emposed requirement to be MonadNix (Execution stage). MonadNix constraint tries to put the cart before horse and seems superflous, since Eval in Nix also needs and can throw exceptions. It is between `nverr` and `evalError`.
-instance MonadNix e t f m => MonadEval (NValue t f m) m where
+instance (MonadNix e f m, t ~ Thunk m) => MonadEval (Free (NValue' f m) t) m where
   freeVariable var =
-    nverr @e @t @f
+    nverr @e @f
       $  ErrorCall
       $  "Undefined variable '"
       <> Text.unpack var
@@ -167,19 +170,19 @@ instance MonadNix e t f m => MonadEval (NValue t f m) m where
   synHole name = do
     span  <- currentPos
     scope <- currentScopes
-    evalError @(NValue t f m) $ SynHole $ SynHoleInfo
+    evalError @(Free (NValue' f m) t) $ SynHole $ SynHoleInfo
       { _synHoleInfo_expr  = Fix $ NSynHole_ span name
       , _synHoleInfo_scope = scope
       }
 
   attrMissing ks Nothing =
-    evalError @(NValue t f m)
+    evalError @(Free (NValue' f m) t)
       $  ErrorCall
       $  "Inheriting unknown attribute: "
       <> intercalate "." (fmap Text.unpack (NE.toList ks))
 
   attrMissing ks (Just s) =
-    evalError @(NValue t f m)
+    evalError @(Free (NValue' f m) t)
       $  ErrorCall
       $  "Could not look up attribute "
       <> intercalate "." (fmap Text.unpack (NE.toList ks))
@@ -189,14 +192,14 @@ instance MonadNix e t f m => MonadEval (NValue t f m) m where
   evalCurPos = do
     scope                  <- currentScopes
     span@(SrcSpan delta _) <- currentPos
-    addProvenance @_ @_ @(NValue t f m)
+    addProvenance
         (Provenance scope (NSym_ span "__curPos"))
       <$> toValue delta
 
   evaledSym name val = do
     scope <- currentScopes
     span  <- currentPos
-    pure $ addProvenance @_ @_ @(NValue t f m)
+    pure $ addProvenance @_ @_ @(Free (NValue' f m) t)
       (Provenance scope (NSym_ span name))
       val
 
@@ -221,12 +224,12 @@ instance MonadNix e t f m => MonadEval (NValue t f m) m where
     scope <- currentScopes
     span  <- currentPos
     nvPathP (Provenance scope (NLiteralPath_ span p))
-      <$> makeAbsolutePath @t @f @m p
+      <$> makeAbsolutePath @f @m p
 
   evalEnvPath p = do
     scope <- currentScopes
     span  <- currentPos
-    nvPathP (Provenance scope (NEnvPath_ span p)) <$> findEnvPath @t @f @m p
+    nvPathP (Provenance scope (NEnvPath_ span p)) <$> findEnvPath @f @m p
 
   evalUnary op arg = do
     scope <- currentScopes
@@ -289,11 +292,11 @@ instance MonadNix e t f m => MonadEval (NValue t f m) m where
 
 infixl 1 `callFunc`
 callFunc
-  :: forall e t f m
-   . MonadNix e t f m
-  => NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  :: forall e f m
+   . MonadNix e f m
+  => NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 callFunc fun arg = demand fun $ \fun' -> do
   frames :: Frames <- asks (view hasLens)
   when (length frames > 2000) $ throwError $ ErrorCall
@@ -303,18 +306,18 @@ callFunc fun arg = demand fun $ \fun' -> do
       f arg
     NVBuiltin name f -> do
       span <- currentPos
-      withFrame Info (Calling @m @(NValue t f m) name span) (f arg)
+      withFrame Info (Calling @m @(Thunk m) name span) (f arg)
     s@(NVSet m _) | Just f <- M.lookup "__functor" m -> do
       demand f $ (`callFunc` s) >=> (`callFunc` arg)
     x -> throwError $ ErrorCall $ "Attempt to call non-function: " <> show x
 
 execUnaryOp
-  :: (Framed e m, MonadCited t f m, Show t)
-  => Scopes m (NValue t f m)
+  :: (Framed e m, MonadCited f m, Show (Thunk m))
+  => Scopes m (NValue f m)
   -> SrcSpan
   -> NUnaryOp
-  -> NValue t f m
-  -> m (NValue t f m)
+  -> NValue f m
+  -> m (NValue f m)
 execUnaryOp scope span op arg = do
   case arg of
     NVConstant c -> case (op, c) of
@@ -336,14 +339,14 @@ execUnaryOp scope span op arg = do
   unaryOp = pure . nvConstantP (Provenance scope (NUnary_ span op (pure arg)))
 
 execBinaryOp
-  :: forall e t f m
-   . (MonadNix e t f m, MonadEval (NValue t f m) m)
-  => Scopes m (NValue t f m)
+  :: forall e f m
+   . (MonadNix e f m, MonadEval (NValue f m) m)
+  => Scopes m (NValue f m)
   -> SrcSpan
   -> NBinaryOp
-  -> NValue t f m
-  -> m (NValue t f m)
-  -> m (NValue t f m)
+  -> NValue f m
+  -> m (NValue f m)
+  -> m (NValue f m)
 
 execBinaryOp scope span op lval rarg = case op of
   NEq   -> rarg >>= \rval -> valueEqM lval rval >>= boolOp rval
@@ -363,7 +366,7 @@ execBinaryOp scope span op lval rarg = case op of
                  execBinaryOpForced scope span op lval' rval'
 
  where
-  toBoolOp :: Maybe (NValue t f m) -> Bool -> m (NValue t f m)
+  toBoolOp :: Maybe (NValue f m) -> Bool -> m (NValue f m)
   toBoolOp r b = pure $ nvConstantP
     (Provenance scope (NBinary_ span op (pure lval) r))
     (NBool b)
@@ -372,14 +375,14 @@ execBinaryOp scope span op lval rarg = case op of
 
 
 execBinaryOpForced
-  :: forall e t f m
-   . (MonadNix e t f m, MonadEval (NValue t f m) m)
-  => Scopes m (NValue t f m)
+  :: forall e f m
+   . (MonadNix e f m, MonadEval (NValue f m) m)
+  => Scopes m (NValue f m)
   -> SrcSpan
   -> NBinaryOp
-  -> NValue t f m
-  -> NValue t f m
-  -> m (NValue t f m)
+  -> NValue f m
+  -> NValue f m
+  -> m (NValue f m)
 
 execBinaryOpForced scope span op lval rval = case op of
   NLt  -> compare (<)
@@ -407,11 +410,11 @@ execBinaryOpForced scope span op lval rval = case op of
       (\rs2 -> nvStrP prov (ls `mappend` rs2))
         <$> coerceToString callFunc CopyToStore CoerceStringy rs
     (NVPath ls, NVStr rs) -> case getStringNoContext rs of
-      Just rs2 -> nvPathP prov <$> makeAbsolutePath @t @f (ls `mappend` Text.unpack rs2)
+      Just rs2 -> nvPathP prov <$> makeAbsolutePath @f (ls `mappend` Text.unpack rs2)
       Nothing -> throwError $ ErrorCall $
         -- data/nix/src/libexpr/eval.cc:1412
         "A string that refers to a store path cannot be appended to a path."
-    (NVPath ls, NVPath rs) -> nvPathP prov <$> makeAbsolutePath @t @f (ls <> rs)
+    (NVPath ls, NVPath rs) -> nvPathP prov <$> makeAbsolutePath @f (ls <> rs)
 
     (ls@NVSet{}, NVStr rs) ->
       (\ls2 -> nvStrP prov (ls2 `mappend` rs))
@@ -429,11 +432,11 @@ execBinaryOpForced scope span op lval rval = case op of
   NApp  -> throwError $ ErrorCall $ "NApp should be handled by evalApp"
 
  where
-  prov :: Provenance m (NValue t f m)
+  prov :: Provenance m (NValue f m)
   prov = Provenance scope (NBinary_ span op (pure lval) (pure rval))
 
   toBool = pure . nvConstantP prov . NBool
-  compare :: (forall a. Ord a => a -> a -> Bool) -> m (NValue t f m)
+  compare :: (forall a. Ord a => a -> a -> Bool) -> m (NValue f m)
   compare op = case (lval, rval) of
     (NVConstant l, NVConstant r) -> toBool $ l `op` r
     (NVStr l, NVStr r) -> toBool $ l `op` r
@@ -442,13 +445,13 @@ execBinaryOpForced scope span op lval rval = case op of
   toInt = pure . nvConstantP prov . NInt
   toFloat = pure . nvConstantP prov . NFloat
 
-  numBinOp :: (forall a. Num a => a -> a -> a) -> m (NValue t f m)
+  numBinOp :: (forall a. Num a => a -> a -> a) -> m (NValue f m)
   numBinOp op = numBinOp' op op
 
   numBinOp'
     :: (Integer -> Integer -> Integer)
     -> (Float -> Float -> Float)
-    -> m (NValue t f m)
+    -> m (NValue f m)
 
   numBinOp' intOp floatOp = case (lval, rval) of
     (NVConstant l, NVConstant r) -> case (l, r) of
@@ -480,7 +483,7 @@ fromStringNoContext ns = case getStringNoContext ns of
   Nothing  -> throwError $ ErrorCall $ "expected string with no context, but got " <> show ns
 
 addTracing
-  :: (MonadNix e t f m, Has e Options, MonadReader Int n, Alternative n)
+  :: (MonadNix e f m, Has e Options, MonadReader Int n, Alternative n)
   => Alg NExprLocF (m a)
   -> Alg NExprLocF (n (m a))
 addTracing k v = do
@@ -504,22 +507,22 @@ addTracing k v = do
       print $ msg rendered <> " ...done"
       pure res
 
-evalExprLoc :: forall e t f m . MonadNix e t f m => NExprLoc -> m (NValue t f m)
+evalExprLoc :: forall e f m . MonadNix e f m => NExprLoc -> m (NValue f m)
 evalExprLoc expr = do
   opts :: Options <- asks (view hasLens)
   if tracing opts
     then join . (`runReaderT` (0 :: Int)) $ adi
       (addTracing phi)
-      (raise (addStackFrames @(NValue t f m) . addSourcePositions))
+      (raise (addStackFrames @(NValue f m) . addSourcePositions))
       expr
-    else adi phi (addStackFrames @(NValue t f m) . addSourcePositions) expr
+    else adi phi (addStackFrames @(NValue f m) . addSourcePositions) expr
  where
   phi = Eval.eval . annotated . getCompose
   raise k f x = ReaderT $ \e -> k (\t -> runReaderT (f t) e) x
 
-exec :: (MonadNix e t f m, MonadInstantiate m) => [String] -> m (NValue t f m)
+exec :: (MonadNix e f m, MonadInstantiate m) => [String] -> m (NValue f m)
 exec args = either throwError evalExprLoc =<< exec' args
 
 nixInstantiateExpr
-  :: (MonadNix e t f m, MonadInstantiate m) => String -> m (NValue t f m)
+  :: (MonadNix e f m, MonadInstantiate m) => String -> m (NValue f m)
 nixInstantiateExpr s = either throwError evalExprLoc =<< instantiateExpr s
diff --git a/src/Nix/Fresh.hs b/src/Nix/Fresh.hs
index 02c7833a3..502df784e 100644
--- a/src/Nix/Fresh.hs
+++ b/src/Nix/Fresh.hs
@@ -29,15 +29,13 @@ import           Control.Monad.Fail
 import           Control.Monad.Reader ( ReaderT(..)
                               , MonadReader(ask)
                               )
-import           Control.Monad.Ref
-                              ( MonadAtomicRef(..)
+import           Control.Monad.Ref    ( MonadAtomicRef(..)
                               , MonadRef(writeRef, readRef)
                               )
 import           Control.Monad.ST     ( ST )
 import           Data.Typeable     ( Typeable )
 
 import           Nix.Var
-import           Nix.Thunk
 
 
 newtype FreshIdT i m a = FreshIdT { unFreshIdT :: ReaderT (Var m i) m a }
@@ -63,20 +61,10 @@ instance MonadTrans (FreshIdT i) where
 instance MonadBase b m => MonadBase b (FreshIdT i m) where
   liftBase = FreshIdT . liftBase
 
-instance
-  ( MonadVar m
-  , Eq i
-  , Ord i
-  , Show i
-  , Enum i
-  , Typeable i
-  )
-  => MonadThunkId (FreshIdT i m)
- where
-  type ThunkId (FreshIdT i m) = i
-  freshId = FreshIdT $ do
-    v <- ask
-    atomicModifyVar v (\i -> (succ i, i))
+freshId :: (Monad m, MonadAtomicRef m, Enum i) => FreshIdT i m i
+freshId = FreshIdT $ do
+  v <- ask
+  atomicModifyVar v (\i -> (succ i, i))
 
 runFreshIdT :: Functor m => Var m i -> FreshIdT i m a -> m a
 runFreshIdT i m = runReaderT (unFreshIdT m) i
diff --git a/src/Nix/Fresh/Basic.hs b/src/Nix/Fresh/Basic.hs
index 579751bc5..e220d8810 100644
--- a/src/Nix/Fresh/Basic.hs
+++ b/src/Nix/Fresh/Basic.hs
@@ -1,52 +1,5 @@
-{-# LANGUAGE CPP #-}
-{-# LANGUAGE FlexibleInstances #-}
-{-# LANGUAGE MultiParamTypeClasses #-}
-{-# LANGUAGE ScopedTypeVariables #-}
-{-# LANGUAGE TypeApplications #-}
-{-# LANGUAGE TypeSynonymInstances #-}
-
-{-# OPTIONS_GHC -Wno-orphans #-}
-
-
 module Nix.Fresh.Basic where
 
-#if !MIN_VERSION_base(4,13,0)
-import           Control.Monad.Fail ( MonadFail )
-#endif
-import           Control.Monad.Reader
-import           Nix.Effects
-import           Nix.Render
-import           Nix.Fresh
-import           Nix.Value
-
-type StdIdT = FreshIdT Int
-
--- NOTE: These would be removed by: https://github.com/haskell-nix/hnix/pull/804
-instance (MonadFail m, MonadFile m) => MonadFile (StdIdT m)
-instance MonadIntrospect m => MonadIntrospect (StdIdT m)
-instance MonadStore m => MonadStore (StdIdT m)
-instance MonadPutStr m => MonadPutStr (StdIdT m)
-instance MonadHttp m => MonadHttp (StdIdT m)
-instance MonadEnv m => MonadEnv (StdIdT m)
-instance MonadPaths m => MonadPaths (StdIdT m)
-instance MonadInstantiate m => MonadInstantiate (StdIdT m)
-instance MonadExec m => MonadExec (StdIdT m)
+import           Nix.Fresh.Stable
 
-instance (MonadEffects t f m, MonadDataContext f m)
-  => MonadEffects t f (StdIdT m) where
-  makeAbsolutePath = lift . makeAbsolutePath @t @f @m
-  findEnvPath      = lift . findEnvPath @t @f @m
-  findPath vs path = do
-    i <- FreshIdT ask
-    let vs' = fmap (unliftNValue (runFreshIdT i)) vs
-    lift $ findPath @t @f @m vs' path
-  importPath path = do
-    i <- FreshIdT ask
-    p <- lift $ importPath @t @f @m path
-    pure $ liftNValue (runFreshIdT i) p
-  pathToDefaultNix = lift . pathToDefaultNix @t @f @m
-  derivationStrict v = do
-    i <- FreshIdT ask
-    p <- lift $ derivationStrict @t @f @m $ unliftNValue (runFreshIdT i) v
-    pure $ liftNValue (runFreshIdT i) p
-  traceEffect = lift . traceEffect @t @f @m
+type StdIdT = FreshStableIdT
diff --git a/src/Nix/Fresh/Stable.hs b/src/Nix/Fresh/Stable.hs
new file mode 100644
index 000000000..fe67ac8ae
--- /dev/null
+++ b/src/Nix/Fresh/Stable.hs
@@ -0,0 +1,102 @@
+{-# LANGUAGE CPP #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE StandaloneDeriving #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# options_ghc -fno-warn-orphans #-} -- TODO MonadTransWrap StateT orphan
+
+module Nix.Fresh.Stable (FreshStableIdT, runFreshStableIdT, freshId) where
+
+import Nix.Effects
+import Nix.Render
+import Nix.Thunk
+import Nix.Thunk.StableId
+import Control.Monad.Reader
+import Control.Monad.State.Strict
+import Control.Monad.Ref
+import Control.Monad.Catch
+import Control.Applicative
+#ifdef MIN_VERSION_haskeline
+import System.Console.Haskeline.MonadException (MonadException)
+#endif
+
+newtype FreshStableIdT m a = FreshStableIdT (ReaderT StableId (StateT Int m) a)
+  deriving
+    ( Functor
+    , Applicative
+    , Monad
+    , MonadRef
+    , MonadAtomicRef
+    , MonadCatch
+    , MonadThrow
+    , MonadIO
+    , MonadFix
+    , MonadPlus
+    , Alternative
+#ifdef MIN_VERSION_haskeline
+    , MonadException
+#endif
+    , MonadMask
+    )
+
+instance MonadState s m => MonadState s (FreshStableIdT m) where
+  get = lift get
+  put = lift . put
+  state = lift . state
+
+instance MonadTrans FreshStableIdT where
+  lift = FreshStableIdT . lift . lift
+
+instance MonadTransWrap (StateT s) where
+  liftWrap f a = do
+    old <- get
+    (result, new) <- lift $ f $ runStateT a old
+    put new
+    pure result
+
+instance MonadTransWrap FreshStableIdT where
+  liftWrap f (FreshStableIdT a) = FreshStableIdT $ liftWrap (liftWrap f) a
+
+runFreshStableIdT :: Monad m => StableId -> FreshStableIdT m a -> m a
+runFreshStableIdT root (FreshStableIdT a) = evalStateT (runReaderT a root) 0
+
+freshId :: Monad m => FreshStableIdT m StableId
+freshId = FreshStableIdT $ do
+  root <- ask
+  n <- get
+  put $ succ n
+  pure $ cons n root
+
+instance MonadFile m => MonadFile (FreshStableIdT m)
+instance MonadIntrospect m => MonadIntrospect (FreshStableIdT m)
+instance MonadStore m => MonadStore (FreshStableIdT m)
+instance MonadPutStr m => MonadPutStr (FreshStableIdT m)
+instance MonadHttp m => MonadHttp (FreshStableIdT m)
+instance MonadEnv m => MonadEnv (FreshStableIdT m)
+instance MonadInstantiate m => MonadInstantiate (FreshStableIdT m)
+instance MonadExec m => MonadExec (FreshStableIdT m)
+deriving instance MonadFail m => MonadFail (FreshStableIdT m)
+
+{-
+instance (MonadEffects t f m, MonadDataContext f m)
+  => MonadEffects t f (FreshStableIdT m) where
+  makeAbsolutePath = lift . makeAbsolutePath @t @f @m
+  findEnvPath      = lift . findEnvPath @t @f @m
+  findPath vs path = do
+    root <- freshId
+    let vs' = map (unliftNValue (runFreshStableIdT root)) vs
+    lift $ findPath @t @f @m vs' path
+  importPath path = do
+    root <- freshId
+    p <- lift $ importPath @t @f @m path
+    pure $ liftNValue (runFreshStableIdT root) p
+  pathToDefaultNix = lift . pathToDefaultNix @t @f @m
+  derivationStrict v = do
+    root <- freshId
+    p <- lift $ derivationStrict @t @f @m (unliftNValue (runFreshStableIdT root) v)
+    pure $ liftNValue (runFreshStableIdT root) p
+  traceEffect = lift . traceEffect @t @f @m
+-}
diff --git a/src/Nix/Json.hs b/src/Nix/Json.hs
index 67ab77019..a43e10416 100644
--- a/src/Nix/Json.hs
+++ b/src/Nix/Json.hs
@@ -1,6 +1,7 @@
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE TypeFamilies #-}
 
 module Nix.Json where
 
@@ -22,7 +23,7 @@ import           Nix.Utils
 import           Nix.Value
 import           Nix.Value.Monad
 
-nvalueToJSONNixString :: MonadNix e t f m => NValue t f m -> m NixString
+nvalueToJSONNixString :: MonadNix e f m => NValue f m -> m NixString
 nvalueToJSONNixString =
   runWithStringContextT
     . fmap
@@ -33,7 +34,7 @@ nvalueToJSONNixString =
         )
     . nvalueToJSON
 
-nvalueToJSON :: MonadNix e t f m => NValue t f m -> WithStringContextT m A.Value
+nvalueToJSON :: MonadNix e f m => NValue f m -> WithStringContextT m A.Value
 nvalueToJSON = \case
   NVConstant (NInt   n) -> pure $ A.toJSON n
   NVConstant (NFloat n) -> pure $ A.toJSON n
diff --git a/src/Nix/Lint.hs b/src/Nix/Lint.hs
index 6dc53c803..4af64cff4 100644
--- a/src/Nix/Lint.hs
+++ b/src/Nix/Lint.hs
@@ -38,11 +38,13 @@ import           Nix.Eval                       ( MonadEval(..) )
 import qualified Nix.Eval                      as Eval
 import           Nix.Expr
 import           Nix.Frames
-import           Nix.Fresh
+import           Nix.Fresh ()
 import           Nix.String
 import           Nix.Options
 import           Nix.Scope
 import           Nix.Thunk
+import           Nix.Thunk.StableId
+import           Nix.Fresh.Stable
 import           Nix.Thunk.Basic
 import           Nix.Utils
 import           Nix.Var
@@ -412,7 +414,7 @@ lintApp context fun arg = unpackSymbolic fun >>= \case
     (head args, ) <$> foldM (unify context) y ys
 
 newtype Lint s a = Lint
-  { runLint :: ReaderT (Context (Lint s) (Symbolic (Lint s))) (FreshIdT Int (ST s)) a }
+  { runLint :: ReaderT (Context (Lint s) (Symbolic (Lint s))) (FreshStableIdT (ST s)) a }
   deriving
     ( Functor
     , Applicative
@@ -432,8 +434,7 @@ instance MonadCatch (Lint s) where
 
 runLintM :: Options -> Lint s a -> ST s a
 runLintM opts action = do
-  i <- newVar (1 :: Int)
-  runFreshIdT i $ flip runReaderT (newContext opts) $ runLint action
+  runFreshStableIdT nil $ flip runReaderT (newContext opts) $ runLint action
 
 symbolicBaseEnv :: Monad m => m (Scopes m (Symbolic m))
 symbolicBaseEnv = pure emptyScopes
diff --git a/src/Nix/Normal.hs b/src/Nix/Normal.hs
index 7ac490674..47ca978f4 100644
--- a/src/Nix/Normal.hs
+++ b/src/Nix/Normal.hs
@@ -6,14 +6,18 @@
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
+{-# LANGUAGE UndecidableInstances #-}
 
 -- | Code for normalization (reduction into a normal form) of Nix expressions.
 -- Nix language allows recursion, so some expressions do not converge.
 -- And so do not converge into a normal form.
 module Nix.Normal where
 
+import           Control.Comonad
 import           Control.Monad
 import           Control.Monad.Free
 import           Control.Monad.Trans.Class
@@ -27,36 +31,37 @@ import           Nix.Thunk
 import           Nix.Value
 import           Nix.Utils
 
-newtype NormalLoop t f m = NormalLoop (NValue t f m)
-    deriving Show
+newtype NormalLoop f m = NormalLoop (NValue f m)
 
-instance MonadDataErrorContext t f m => Exception (NormalLoop t f m)
+deriving instance (Comonad f, Show (Thunk m)) => Show (NormalLoop f m)
+
+instance MonadDataErrorContext f m => Exception (NormalLoop f m)
 
 -- | Normalize the value as much as possible, leaving only detected cycles.
 normalizeValue
   :: forall e t m f
    . ( Framed e m
-     , MonadThunk t m (NValue t f m)
-     , MonadDataErrorContext t f m
-     , Ord (ThunkId m)
+     , MonadThunk m, Thunk m ~ t, ThunkValue m ~ NValue f m
+     , MonadDataErrorContext f m
+     , Ord (Thunk m)
      )
-  => (forall r . t -> (NValue t f m -> m r) -> m r)
-  -> NValue t f m
-  -> m (NValue t f m)
+  => (forall r . t -> (NValue f m -> m r) -> m r)
+  -> NValue f m
+  -> m (NValue f m)
 normalizeValue f = run . iterNValueM run go (fmap Free . sequenceNValue' run)
  where
   start = 0 :: Int
   table = mempty
 
-  run :: ReaderT Int (StateT (Set (ThunkId m)) m) r -> m r
+  run :: ReaderT Int (StateT (Set (Thunk m)) m) r -> m r
   run = (`evalStateT` table) . (`runReaderT` start)
 
   go
     :: t
-    -> (  NValue t f m
-       -> ReaderT Int (StateT (Set (ThunkId m)) m) (NValue t f m)
+    -> (  NValue f m
+       -> ReaderT Int (StateT (Set (Thunk m)) m) (NValue f m)
        )
-    -> ReaderT Int (StateT (Set (ThunkId m)) m) (NValue t f m)
+    -> ReaderT Int (StateT (Set (Thunk m)) m) (NValue f m)
   go t k = do
     b <- seen t
     if b
@@ -68,66 +73,65 @@ normalizeValue f = run . iterNValueM run go (fmap Free . sequenceNValue' run)
         lifted (lifted (f t)) $ local succ . k
 
   seen t = do
-    let tid = thunkId t
     lift $ do
-      res <- gets (member tid)
-      unless res $ modify (insert tid)
+      res <- gets (member t)
+      unless res $ modify (insert t)
       pure res
 
 normalForm
   :: ( Framed e m
-     , MonadThunk t m (NValue t f m)
-     , MonadDataErrorContext t f m
-     , HasCitations m (NValue t f m) t
-     , HasCitations1 m (NValue t f m) f
-     , Ord (ThunkId m)
+     , MonadThunk m, ThunkValue m ~ NValue f m
+     , MonadDataErrorContext f m
+     , HasCitations m (NValue f m) (Thunk m)
+     , HasCitations1 m (NValue f m) f
+     , Ord (Thunk m)
      )
-  => NValue t f m
-  -> m (NValue t f m)
+  => NValue f m
+  -> m (NValue f m)
 normalForm = fmap stubCycles . normalizeValue force
 
 normalForm_
   :: ( Framed e m
-     , MonadThunk t m (NValue t f m)
-     , MonadDataErrorContext t f m
-     , Ord (ThunkId m)
+     , MonadThunk m, ThunkValue m ~ NValue f m
+     , MonadDataErrorContext f m
+     , Ord (Thunk m)
      )
-  => NValue t f m
+  => NValue f m
   -> m ()
 normalForm_ = void <$> normalizeValue forceEff
 
 stubCycles
-  :: forall t f m
+  :: forall f m
    . ( MonadDataContext f m
-     , HasCitations m (NValue t f m) t
-     , HasCitations1 m (NValue t f m) f
+     , HasCitations m (NValue f m) (Thunk m)
+     , HasCitations1 m (NValue f m) f
      )
-  => NValue t f m
-  -> NValue t f m
+  => NValue f m
+  -> NValue f m
 stubCycles = flip iterNValue Free $ \t _ ->
   Free
     $ NValue
-    $ Prelude.foldr (addProvenance1 @m @(NValue t f m)) cyc
+    $ Prelude.foldr (addProvenance1 @m @(NValue f m)) cyc
     $ reverse
-    $ citations @m @(NValue t f m) t
+    $ citations @m @(NValue f m) t
  where
   Free (NValue cyc) = opaque
 
 removeEffects
-  :: (MonadThunk t m (NValue t f m), MonadDataContext f m)
-  => NValue t f m
-  -> m (NValue t f m)
+  :: (MonadThunk m, ThunkValue m ~ NValue f m, MonadDataContext f m)
+  => NValue f m
+  -> m (NValue f m)
 removeEffects =
   iterNValueM
     id
     (`queryM` pure opaque)
     (fmap Free . sequenceNValue' id)
 
-opaque :: Applicative f => NValue t f m
+opaque :: Applicative f => NValue f m
 opaque = nvStr $ makeNixStringWithoutContext "<expr>"
 
 dethunk
-  :: (MonadThunk t m (NValue t f m), MonadDataContext f m)
+  :: (MonadThunk m, Thunk m ~ t, ThunkValue m ~ NValue f m, MonadDataContext f m)
   => t
-  -> m (NValue t f m)
+  -> m (NValue f m)
 dethunk t = queryM t (pure opaque) removeEffects
diff --git a/src/Nix/Pretty.hs b/src/Nix/Pretty.hs
index bb94b96fe..6ce64f666 100644
--- a/src/Nix/Pretty.hs
+++ b/src/Nix/Pretty.hs
@@ -8,6 +8,7 @@
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE ViewPatterns #-}
+{-# LANGUAGE TypeFamilies #-}
 
 {-# OPTIONS_GHC -fno-warn-name-shadowing #-}
 
@@ -185,16 +186,16 @@ prettyNix :: NExpr -> Doc ann
 prettyNix = withoutParens . foldFix exprFNixDoc
 
 prettyOriginExpr
-  :: forall t f m ann
-   . HasCitations1 m (NValue t f m) f
-  => NExprLocF (Maybe (NValue t f m))
+  :: forall f m ann
+   . HasCitations1 m (NValue f m) f
+  => NExprLocF (Maybe (NValue f m))
   -> Doc ann
 prettyOriginExpr = withoutParens . go
  where
   go = exprFNixDoc . annotated . getCompose . fmap render
 
-  render :: Maybe (NValue t f m) -> NixDoc ann
-  render Nothing = simpleExpr "_"
+  render :: Maybe (NValue f m) -> NixDoc ann
+  render Nothing = simpleExpr $ "_"
   render (Just (Free (reverse . citations @m -> p:_))) = go (_originExpr p)
   render _       = simpleExpr "?"
     -- render (Just (NValue (citations -> ps))) =
@@ -300,12 +301,12 @@ exprFNixDoc = \case
   NSynHole name -> simpleExpr $ pretty ("^" <> unpack name)
   where recPrefix = "rec" <> space
 
-valueToExpr :: forall t f m . MonadDataContext f m => NValue t f m -> NExpr
+valueToExpr :: forall f m . MonadDataContext f m => NValue f m -> NExpr
 valueToExpr = iterNValue (\_ _ -> thk) phi
  where
   thk = Fix . NSym . pack $ "<expr>"
 
-  phi :: NValue' t f m NExpr -> NExpr
+  phi :: NValue' f m NExpr -> NExpr
   phi (NVConstant' a ) = Fix $ NConstant a
   phi (NVStr'      ns) = mkStr ns
   phi (NVList'     l ) = Fix $ NList l
@@ -321,20 +322,20 @@ valueToExpr = iterNValue (\_ _ -> thk) phi
   mkStr ns = Fix $ NStr $ DoubleQuoted [Plain (stringIgnoreContext ns)]
 
 prettyNValue
-  :: forall t f m ann . MonadDataContext f m => NValue t f m -> Doc ann
+  :: forall f m ann . MonadDataContext f m => NValue f m -> Doc ann
 prettyNValue = prettyNix . valueToExpr
 
 prettyNValueProv
   :: forall t f m ann
-   . ( HasCitations m (NValue t f m) t
-     , HasCitations1 m (NValue t f m) f
-     , MonadThunk t m (NValue t f m)
+   . ( HasCitations m (NValue f m) t
+     , HasCitations1 m (NValue f m) f
+     , MonadThunk m, Thunk m ~ t, ThunkValue m ~ NValue f m
      , MonadDataContext f m
      )
-  => NValue t f m
+  => NValue f m
   -> Doc ann
 prettyNValueProv v = do
-  let ps = citations @m @(NValue t f m) v
+  let ps = citations @m @(NValue f m) v
   case ps of
     [] -> prettyNValue v
     ps ->
@@ -350,15 +351,15 @@ prettyNValueProv v = do
 
 prettyNThunk
   :: forall t f m ann
-   . ( HasCitations m (NValue t f m) t
-     , HasCitations1 m (NValue t f m) f
-     , MonadThunk t m (NValue t f m)
+   . ( HasCitations m (NValue f m) t
+     , HasCitations1 m (NValue f m) f
+     , MonadThunk m, Thunk m ~ t, ThunkValue m ~ NValue f m
      , MonadDataContext f m
      )
   => t
   -> m (Doc ann)
 prettyNThunk t = do
-  let ps = citations @m @(NValue t f m) @t t
+  let ps = citations @m @(NValue f m) @t t
   v' <- prettyNValue <$> dethunk t
   pure
     $ fillSep
@@ -371,12 +372,12 @@ prettyNThunk t = do
       ]
 
 -- | This function is used only by the testing code.
-printNix :: forall t f m . MonadDataContext f m => NValue t f m -> String
+printNix :: forall f m . MonadDataContext f m => NValue f m -> String
 printNix = iterNValue (\_ _ -> thk) phi
  where
   thk = "<thunk>"
 
-  phi :: NValue' t f m String -> String
+  phi :: NValue' f m String -> String
   phi (NVConstant' a ) = unpack $ atomText a
   phi (NVStr'      ns) = show $ stringIgnoreContext ns
   phi (NVList'     l ) = "[ " <> unwords l <> " ]"
diff --git a/src/Nix/Reduce.hs b/src/Nix/Reduce.hs
index fa9d9b1e4..093364b94 100644
--- a/src/Nix/Reduce.hs
+++ b/src/Nix/Reduce.hs
@@ -76,7 +76,7 @@ newtype Reducer m a = Reducer
 staticImport
   :: forall m
    . ( MonadIO m
-     , Scoped NExprLoc m
+     , Scoped m NExprLoc m
      , MonadFail m
      , MonadReader (Maybe FilePath, Scopes m NExprLoc) m
      , MonadState (HashMap FilePath NExprLoc, HashMap Text Text) m
@@ -131,7 +131,7 @@ reduceExpr mpath expr =
 reduce
   :: forall m
    . ( MonadIO m
-     , Scoped NExprLoc m
+     , Scoped m NExprLoc m
      , MonadFail m
      , MonadReader (Maybe FilePath, Scopes m NExprLoc) m
      , MonadState (HashMap FilePath NExprLoc, MS.HashMap Text Text) m
@@ -220,18 +220,18 @@ reduce e@(NSet_ ann NNonRecursive binds) = do
         Inherit{} -> True
         _         -> False
   if usesInherit
-    then clearScopes @NExprLoc $ Fix . NSet_ ann NNonRecursive <$> traverse sequence binds
+    then clearScopes $ Fix . NSet_ ann NNonRecursive <$> traverse sequence binds
     else Fix <$> sequence e
 
 -- Encountering a 'rec set' construction eliminates any hope of inlining
 -- definitions.
 reduce (NSet_ ann NRecursive binds) =
-  clearScopes @NExprLoc $ Fix . NSet_ ann NRecursive <$> traverse sequence binds
+  clearScopes $ Fix . NSet_ ann NRecursive <$> traverse sequence binds
 
 -- Encountering a 'with' construction eliminates any hope of inlining
 -- definitions.
 reduce (NWith_ ann scope body) =
-  clearScopes @NExprLoc $ fmap Fix $ NWith_ ann <$> scope <*> body
+  clearScopes $ fmap Fix $ NWith_ ann <$> scope <*> body
 
 -- | Reduce a let binds section by pushing lambdas,
 --   constants and strings to the body scope.
@@ -414,8 +414,8 @@ reducingEvalExpr eval mpath expr = do
   pure (fromMaybe nNull expr'', eres)
   where addEvalFlags k (FlaggedF (b, x)) = liftIO (writeIORef b True) *> k x
 
-instance Monad m => Scoped NExprLoc (Reducer m) where
+instance Monad m => Scoped (Reducer m) NExprLoc (Reducer m) where
   currentScopes = currentScopesReader
   clearScopes   = clearScopesReader @(Reducer m) @NExprLoc
   pushScopes    = pushScopesReader
-  lookupVar     = lookupVarReader
+  askLookupVar  = lookupVarReader
diff --git a/src/Nix/Render.hs b/src/Nix/Render.hs
index c4095c1b0..8ea6c585a 100644
--- a/src/Nix/Render.hs
+++ b/src/Nix/Render.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE UndecidableInstances #-}
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE ConstraintKinds #-}
@@ -9,6 +10,7 @@
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE ViewPatterns #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
 
 module Nix.Render where
 
@@ -24,8 +26,7 @@ import qualified Data.Set                      as Set
 import qualified Data.Text                     as T
 import qualified Data.Text.Encoding            as T
 import           Data.Void
-import           Nix.Utils.Fix1                 ( Fix1T
-                                                , MonadFix1T )
+import           Nix.Utils.Fix1
 import           Nix.Expr.Types.Annotated
 import           Prettyprinter
 import qualified System.Directory              as S
@@ -73,8 +74,7 @@ instance MonadFile IO where
   doesDirectoryExist    = S.doesDirectoryExist
   getSymbolicLinkStatus = S.getSymbolicLinkStatus
 
-
-instance (MonadFix1T t m, MonadFail (Fix1T t m), MonadFile m) => MonadFile (Fix1T t m)
+deriving instance MonadFile (t (Fix1T t m) m) => MonadFile (Fix1T t m)
 
 posAndMsg :: SourcePos -> Doc a -> ParseError s Void
 posAndMsg (SourcePos _ lineNo _) msg = FancyError
diff --git a/src/Nix/Render/Frame.hs b/src/Nix/Render/Frame.hs
index 71e24ccdd..b9a0a699b 100644
--- a/src/Nix/Render/Frame.hs
+++ b/src/Nix/Render/Frame.hs
@@ -87,9 +87,9 @@ renderFrame
 renderFrame (NixFrame level f)
   | Just (e :: EvalFrame m v) <- fromException f = renderEvalFrame level e
   | Just (e :: ThunkLoop) <- fromException f = renderThunkLoop level e
-  | Just (e :: ValueFrame t f m) <- fromException f = renderValueFrame level e
-  | Just (e :: NormalLoop t f m) <- fromException f = renderNormalLoop level e
-  | Just (e :: ExecFrame t f m) <- fromException f = renderExecFrame level e
+  | Just (e :: ValueFrame f m) <- fromException f = renderValueFrame level e
+  | Just (e :: NormalLoop f m) <- fromException f = renderNormalLoop level e
+  | Just (e :: ExecFrame f m) <- fromException f = renderExecFrame level e
   | Just (e :: ErrorCall) <- fromException f = pure [pretty (show e)]
   | Just (e :: SynHoleInfo m v) <- fromException f = pure [pretty (show e)]
   | otherwise = error $ "Unrecognized frame: " <> show f
@@ -161,7 +161,7 @@ renderValueFrame
   :: forall e t f m ann
    . (MonadReader e m, Has e Options, MonadFile m, MonadCitedThunks t f m)
   => NixLevel
-  -> ValueFrame t f m
+  -> ValueFrame f m
   -> m [Doc ann]
 renderValueFrame level = fmap (: mempty) . \case
   ForcingThunk    _t -> pure "ForcingThunk" -- jww (2019-03-18): NYI
@@ -191,7 +191,7 @@ renderValue
   => NixLevel
   -> String
   -> String
-  -> NValue t f m
+  -> NValue f m
   -> m (Doc ann)
 renderValue _level _longLabel _shortLabel v = do
   opts :: Options <- asks (view hasLens)
@@ -202,7 +202,7 @@ renderValue _level _longLabel _shortLabel v = do
 renderExecFrame
   :: (MonadReader e m, Has e Options, MonadFile m, MonadCitedThunks t f m)
   => NixLevel
-  -> ExecFrame t f m
+  -> ExecFrame f m
   -> m [Doc ann]
 renderExecFrame level = \case
   Assertion ann v ->
@@ -213,7 +213,7 @@ renderExecFrame level = \case
           )
 
 renderThunkLoop
-  :: (MonadReader e m, Has e Options, MonadFile m, Show (ThunkId m))
+  :: (MonadReader e m, Has e Options, MonadFile m, Show (Thunk m))
   => NixLevel
   -> ThunkLoop
   -> m [Doc ann]
@@ -223,7 +223,7 @@ renderThunkLoop _level = pure . (: mempty) . \case
 renderNormalLoop
   :: (MonadReader e m, Has e Options, MonadFile m, MonadCitedThunks t f m)
   => NixLevel
-  -> NormalLoop t f m
+  -> NormalLoop f m
   -> m [Doc ann]
 renderNormalLoop level = fmap (: mempty) . \case
   NormalLoop v -> do
diff --git a/src/Nix/Scope.hs b/src/Nix/Scope.hs
index d3c848a44..6aa3015e1 100644
--- a/src/Nix/Scope.hs
+++ b/src/Nix/Scope.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE AllowAmbiguousTypes #-}
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE DeriveTraversable #-}
@@ -7,15 +8,29 @@
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE UndecidableInstances #-}
 
 module Nix.Scope where
 
 import           Control.Applicative
 import           Control.Monad.Reader
+import           Control.Monad.Trans.Writer
 import qualified Data.HashMap.Lazy             as M
 import           Data.Text                      ( Text )
 import           Lens.Family2
 import           Nix.Utils
+import           Nix.Utils.Fix1
+
+class Scoped r binding m | m -> binding r where
+  currentScopes :: m (Scopes r binding)
+  clearScopes :: m x -> m x
+  pushScopes :: Scopes r binding -> m x -> m x
+  askLookupVar :: Text -> m (r (Maybe binding))
+
+deriving instance Scoped r a (t (Fix1T t m) m) => Scoped r a (Fix1T t m)
+
+lookupVar :: (Monad m, Scoped m binding m) => Text -> m (Maybe binding)
+lookupVar = join . askLookupVar
 
 newtype Scope a = Scope { getScope :: AttrSet a }
     deriving (Functor, Foldable, Traversable, Eq)
@@ -48,15 +63,12 @@ instance Monoid (Scopes m a) where
   mempty  = emptyScopes
   mappend = (<>)
 
+instance Functor m => Functor (Scopes m) where
+  fmap f (Scopes l d) = Scopes (fmap (fmap f) l) (fmap (fmap (fmap f)) d)
+
 emptyScopes :: forall m a . Scopes m a
 emptyScopes = Scopes mempty mempty
 
-class Scoped a m | m -> a where
-  currentScopes :: m (Scopes m a)
-  clearScopes :: m r -> m r
-  pushScopes :: Scopes m a -> m r -> m r
-  lookupVar :: Text -> m (Maybe a)
-
 currentScopesReader
   :: forall m a e . (MonadReader e m, Has e (Scopes m a)) => m (Scopes m a)
 currentScopesReader = asks (view hasLens)
@@ -65,24 +77,33 @@ clearScopesReader
   :: forall m a e r . (MonadReader e m, Has e (Scopes m a)) => m r -> m r
 clearScopesReader = local (set hasLens (emptyScopes @m @a))
 
-pushScope :: Scoped a m => AttrSet a -> m r -> m r
+strongScope :: AttrSet a -> Scopes m a
+strongScope a = Scopes [Scope a] mempty
+
+pushScope :: Scoped r a m => AttrSet a -> m x -> m x
 pushScope s = pushScopes (Scopes [Scope s] mempty)
 
-pushWeakScope :: (Functor m, Scoped a m) => m (AttrSet a) -> m r -> m r
+pushWeakScope :: (Functor r, Scoped r a m) => r (AttrSet a) -> m x -> m x
 pushWeakScope s = pushScopes (Scopes mempty [Scope <$> s])
 
 pushScopesReader
-  :: (MonadReader e m, Has e (Scopes m a)) => Scopes m a -> m r -> m r
+  :: (MonadReader e m, Has e (Scopes r a)) => Scopes r a -> m x -> m x
 pushScopesReader s = local (over hasLens (s <>))
 
 lookupVarReader
-  :: forall m a e . (MonadReader e m, Has e (Scopes m a)) => Text -> m (Maybe a)
+  :: forall r m a e . (Monad r, MonadReader e m, Has e (Scopes r a)) => Text -> m (r (Maybe a))
 lookupVarReader k = do
-  mres <- asks (scopeLookup k . lexicalScopes @m . view hasLens)
+  s <- asks $ view hasLens
+  pure $ lookupVarScopes k s
+
+lookupVarScopes
+  :: forall m a . Monad m => Text -> Scopes m a -> m (Maybe a)
+lookupVarScopes k s = do
+  let mres = scopeLookup k $ lexicalScopes @m s
   case mres of
     Just sym -> pure $ pure sym
     Nothing  -> do
-      ws <- asks (dynamicScopes . view hasLens)
+      let ws = dynamicScopes s
       foldr
         (\x rest -> do
           mres' <- M.lookup k . getScope <$> x
@@ -93,5 +114,20 @@ lookupVarReader k = do
         (pure Nothing)
         ws
 
-withScopes :: Scoped a m => Scopes m a -> m r -> m r
+withScopes :: Scoped r a m => Scopes r a -> m x -> m x
 withScopes scope = clearScopes . pushScopes scope
+
+hoistDynamicScopes :: (m (Scope a) -> n (Scope a)) -> Scopes m a -> Scopes n a
+hoistDynamicScopes f (Scopes s d) = Scopes s $ fmap f d
+
+instance (Scoped r a m, Monoid w, Monad m) => Scoped r a (WriterT w m) where
+  currentScopes = lift currentScopes
+  clearScopes m = do
+    (a, w) <- lift $ clearScopes $ runWriterT m
+    tell w
+    pure a
+  pushScopes s m = do
+    (a, w) <- lift $ pushScopes s $ runWriterT m
+    tell w
+    pure a
+  askLookupVar = lift . askLookupVar
diff --git a/src/Nix/Scope/Basic.hs b/src/Nix/Scope/Basic.hs
new file mode 100644
index 000000000..dd7f30431
--- /dev/null
+++ b/src/Nix/Scope/Basic.hs
@@ -0,0 +1,92 @@
+{-# LANGUAGE CPP #-}
+{-# LANGUAGE DeriveFunctor #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE ApplicativeDo #-}
+{-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE DefaultSignatures #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE FunctionalDependencies #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE TypeOperators #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE StandaloneDeriving #-}
+module Nix.Scope.Basic where
+
+import Control.Applicative
+import Control.Monad.Exception
+import Control.Monad.Reader
+import Control.Monad.State
+import Control.Monad.Catch
+import Nix.Thunk
+import Nix.Scope
+#ifdef MIN_VERSION_haskeline
+import           System.Console.Haskeline.MonadException hiding(catch)
+#endif
+
+-- `binding` is the information associated with a variable name in the scope
+newtype ScopeT binding r m a = ScopeT { unScopeT :: ReaderT (Scopes r binding) m a }
+  deriving
+    ( Functor
+    , Applicative
+    , Monad
+    , Alternative
+    , MonadPlus
+    , MonadFail
+    , MonadFix
+    , MonadIO
+    , MonadCatch
+    , MonadThrow
+    , MonadException
+    , MonadMask
+    )
+
+deriving instance MonadState s m => MonadState s (ScopeT binding r m)
+
+instance MonadReader a m => MonadReader a (ScopeT binding r m) where
+  ask = lift ask
+  local f = liftWrap $ local f
+  reader = lift . reader
+
+runScopeT :: ScopeT binding r m a -> Scopes r binding -> m a
+runScopeT = runReaderT . unScopeT
+
+instance MonadTrans (ScopeT t r) where
+  lift = ScopeT . lift
+
+instance (Monad m, Monad r) => Scoped r t (ScopeT t r m) where
+  currentScopes = ScopeT ask
+  clearScopes = ScopeT . local (const mempty) . unScopeT
+  pushScopes added = 
+    ScopeT .
+    local (\old -> added <> old) .
+    unScopeT
+  askLookupVar name = ScopeT $ do
+    scopes <- ask
+    pure $ lookupVarScopes name scopes
+
+instance MonadThunk m => MonadThunk (ScopeT binding r m) where
+  type Thunk (ScopeT binding r m) = Thunk m
+  type ThunkValue (ScopeT binding r m) = ThunkValue m
+  thunk a = ScopeT $ do
+    scopes <- ask
+    lift $ thunk $ runScopeT a scopes
+  queryM t n k = ScopeT $ do
+    scopes <- ask
+    lift $ queryM t (runScopeT n scopes) ((`runScopeT` scopes) . k)
+  force t k = ScopeT $ do
+    scopes <- ask
+    lift $ force t $ (`runScopeT` scopes) . k
+  forceEff t k = ScopeT $ do
+    scopes <- ask
+    lift $ forceEff t $ (`runScopeT` scopes) . k
+  further t k = ScopeT $ do
+    scopes <- ask
+    lift $ further t $ (`runScopeT` scopes) . k . lift
+
+instance MonadTransWrap (ScopeT binding r) where
+  liftWrap f a = ScopeT $ liftWrap f (unScopeT a)
diff --git a/src/Nix/Standard.hs b/src/Nix/Standard.hs
index 18fc06cac..eaf7b1125 100644
--- a/src/Nix/Standard.hs
+++ b/src/Nix/Standard.hs
@@ -4,13 +4,16 @@
 {-# LANGUAGE DeriveFoldable #-}
 {-# LANGUAGE DeriveFunctor #-}
 {-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE DeriveTraversable #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE QuantifiedConstraints #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE StandaloneDeriving #-}
 
 {-# OPTIONS_GHC -Wno-orphans #-}
 
@@ -18,8 +21,6 @@
 module Nix.Standard where
 
 import           Control.Applicative
-import           Control.Comonad                ( Comonad )
-import           Control.Comonad.Env            ( ComonadEnv )
 import           Control.Monad.Catch     hiding ( catchJust )
 #if !MIN_VERSION_base(4,13,0)
 import           Control.Monad.Fail             ( MonadFail )
@@ -28,71 +29,34 @@ import           Control.Monad.Free
 import           Control.Monad.Reader
 import           Control.Monad.Ref
 import           Control.Monad.State
+import           Data.Coerce
+import           Data.Functor.Identity
 import           Data.HashMap.Lazy              ( HashMap )
-import qualified Data.HashMap.Strict
 import           Data.Text                      ( Text )
 import           Data.Typeable
-import           GHC.Generics
 import           Nix.Cited
-import           Nix.Cited.Basic
 import           Nix.Context
 import           Nix.Effects
 import           Nix.Effects.Basic
 import           Nix.Effects.Derivation
 import           Nix.Expr.Types.Annotated
-import           Nix.Fresh
-import           Nix.Fresh.Basic
+import           Nix.Thunk.StableId
+import           Nix.Thunk.Basic
 import           Nix.Options
 import           Nix.Render
 import           Nix.Scope
+import           Nix.Scope.Basic
 import           Nix.Thunk
-import           Nix.Thunk.Basic
 import           Nix.Utils.Fix1
 import           Nix.Value
 import           Nix.Value.Monad
-import           Nix.Var
-
-
-newtype StdCited m a = StdCited
-  { _stdCited :: Cited (StdThunk m) (StdCited m) m a }
-  deriving
-    ( Generic
-    , Typeable
-    , Functor
-    , Applicative
-    , Foldable
-    , Traversable
-    , Comonad
-    , ComonadEnv [Provenance m (StdValue m)]
-    )
 
-newtype StdThunk (m :: * -> *) = StdThunk
-  { _stdThunk :: StdCited m (NThunkF m (StdValue m)) }
-
-type StdValue m = NValue (StdThunk m) (StdCited m) m
-
-instance Show (StdThunk m) where
-  show _ = "<thunk>"
-
-instance HasCitations1 m (StdValue m) (StdCited m) where
-  citations1 (StdCited c) = citations1 c
-  addProvenance1 x (StdCited c) = StdCited (addProvenance1 x c)
-
-instance HasCitations m (StdValue m) (StdThunk m) where
-  citations (StdThunk c) = citations1 c
-  addProvenance x (StdThunk c) = StdThunk (addProvenance1 x c)
-
-instance MonadReader (Context m (StdValue m)) m => Scoped (StdValue m) m where
-  currentScopes = currentScopesReader
-  clearScopes   = clearScopesReader @m @(StdValue m)
-  pushScopes    = pushScopesReader
-  lookupVar     = lookupVarReader
 
+instance MonadFile m => MonadFile (StandardTF r m)
 instance ( MonadFix m
          , MonadFile m
          , MonadCatch m
          , MonadEnv m
-         , MonadPaths m
          , MonadExec m
          , MonadHttp m
          , MonadInstantiate m
@@ -102,14 +66,9 @@ instance ( MonadFix m
          , MonadStore m
          , MonadAtomicRef m
          , Typeable m
-         , Scoped (StdValue m) m
-         , MonadReader (Context m (StdValue m)) m
-         , MonadState (HashMap FilePath NExprLoc, Data.HashMap.Strict.HashMap Text Text) m
-         , MonadDataErrorContext (StdThunk m) (StdCited m) m
-         , MonadThunk (StdThunk m) m (StdValue m)
-         , MonadValue (StdValue m) m
+         , MonadPaths m
          )
-  => MonadEffects (StdThunk m) (StdCited m) m where
+  => MonadEffects Identity (StandardT m) where
   makeAbsolutePath = defaultMakeAbsolutePath
   findEnvPath      = defaultFindEnvPath
   findPath         = defaultFindPath
@@ -118,35 +77,6 @@ instance ( MonadFix m
   derivationStrict = defaultDerivationStrict
   traceEffect      = defaultTraceEffect
 
-instance ( MonadAtomicRef m
-         , MonadCatch m
-         , Typeable m
-         , MonadReader (Context m (StdValue m)) m
-         , MonadThunkId m
-         )
-  => MonadThunk (StdThunk m) m (StdValue m) where
-  thunk   = fmap (StdThunk . StdCited) . thunk
-  thunkId = thunkId . _stdCited . _stdThunk
-  queryM x b f = queryM (_stdCited (_stdThunk x)) b f
-  force    = force . _stdCited . _stdThunk
-  forceEff = forceEff . _stdCited . _stdThunk
-  further  = (fmap (StdThunk . StdCited) .) . further . _stdCited . _stdThunk
-
-instance ( MonadAtomicRef m
-         , MonadCatch m
-         , Typeable m
-         , MonadReader (Context m (StdValue m)) m
-         , MonadThunkId m
-         )
-  => MonadValue (StdValue m) m where
-  defer = fmap Pure . thunk
-
-  demand (Pure v) f = force v (flip demand f)
-  demand (Free v) f = f (Free v)
-
-  inform (Pure t) f = Pure <$> further t f
-  inform (Free v) f = Free <$> bindNValue' id (flip inform f) v
-
 {------------------------------------------------------------------------}
 
 -- jww (2019-03-22): NYI
@@ -155,12 +85,15 @@ instance ( MonadAtomicRef m
 -- whileForcingThunk frame =
 --   withFrame Debug (ForcingThunk @t @f @m) . withFrame Debug frame
 
+type StandardTFInner r m = ScopeT (NValue Identity r) r
+  (ThunkT (NValue Identity r) --TODO: What should this `Identity` be? Probably (StdCited ...)
+    (ReaderT Context
+      (StateT (HashMap FilePath NExprLoc, HashMap Text Text) m)))
+
 newtype StandardTF r m a
-  = StandardTF (ReaderT (Context r (StdValue r))
-                        (StateT (HashMap FilePath NExprLoc, HashMap Text Text) m) a)
+  = StandardTF { unStandardTF :: StandardTFInner r m a }
   deriving
-    ( Functor
-    , Applicative
+    ( Applicative
     , Alternative
     , Monad
     , MonadFail
@@ -170,55 +103,93 @@ newtype StandardTF r m a
     , MonadCatch
     , MonadThrow
     , MonadMask
-    , MonadReader (Context r (StdValue r))
+    , MonadReader Context
     , MonadState (HashMap FilePath NExprLoc, HashMap Text Text)
+    , MonadStore
     )
 
+deriving instance (Monad m, Monad r, Thunk r ~ StdThunk r m) => Scoped r (Free (NValue' Identity r) (StdThunk r m)) (StandardTF r m)
+
+deriving instance Functor m => Functor (StandardTF r m)
+
 instance MonadTrans (StandardTF r) where
-  lift = StandardTF . lift . lift
+  lift = StandardTF . lift . lift . lift . lift
+
+instance MonadTransWrap (StandardTF r) where
+  liftWrap f (StandardTF a) = StandardTF $ liftWrap (liftWrap (liftWrap (liftWrap f))) a
+
+instance (MonadPutStr m) => MonadPutStr (StandardTF r m)
+instance (MonadHttp m) => MonadHttp (StandardTF r m)
+instance (MonadEnv m) => MonadEnv (StandardTF r m)
+instance (MonadInstantiate m) => MonadInstantiate (StandardTF r m)
+instance (MonadExec m) => MonadExec (StandardTF r m)
+instance (MonadIntrospect m) => MonadIntrospect (StandardTF r m)
+
+instance ( Monad m
+         , Typeable r
+         , Typeable (Thunk r)
+         , Typeable m
+         , MonadAtomicRef m
+         , MonadCatch m
+         ) => MonadThunk (StandardTF r m) where
+  type Thunk (StandardTF r m) = StdThunk r m
+  type ThunkValue (StandardTF r m) = StdValue r
+  thunk v = StandardTF $ StdThunk <$> thunk (unStandardTF v)
+  queryM = coerce $ queryM @(StandardTFInner r m)
+  force = coerce $ force @(StandardTFInner r m)
+  forceEff = coerce $ forceEff @(StandardTFInner r m)
+  further t f = fmap StdThunk $ StandardTF $ further (unStdThunk t) $ unStandardTF . f . StandardTF
+
+newtype StdThunk r m = StdThunk { unStdThunk :: Thunk (StandardTFInner r m) }
+  deriving (Eq, Ord, Show, Typeable)
+
+type StdValue r = NValue Identity r
 
-instance (MonadPutStr r, MonadPutStr m) => MonadPutStr (StandardTF r m)
-instance (MonadHttp r, MonadHttp m) => MonadHttp (StandardTF r m)
-instance (MonadEnv r, MonadEnv m) => MonadEnv (StandardTF r m)
-instance (MonadPaths r, MonadPaths m) => MonadPaths (StandardTF r m)
-instance (MonadInstantiate r, MonadInstantiate m) => MonadInstantiate (StandardTF r m)
-instance (MonadExec r, MonadExec m) => MonadExec (StandardTF r m)
-instance (MonadIntrospect r, MonadIntrospect m) => MonadIntrospect (StandardTF r m)
+instance MonadPaths m => MonadPaths (StandardTF r m)
+
+instance ( Monad m
+         , Typeable m
+         , MonadAtomicRef m
+         , MonadCatch m
+         ) => MonadValue (Free (NValue' Identity (StandardT m)) (StdThunk (StandardT m) m)) (StandardT m) where
+  defer = fmap pure . thunk
+
+  demand (Pure v) f = force v (`demand` f)
+  demand (Free v) f = f (Free v)
+
+  inform (Pure t) f = Pure <$> further t f
+  inform (Free v) f = Free <$> bindNValue' id (`inform` f) v
+
+--TODO
+instance HasCitations m' v (StdThunk r m) where
+  citations _ = []
+  addProvenance _ = id
+
+instance HasCitations1 m v Identity where
+  citations1 _ = []
+  addProvenance1 _ = id
 
 ---------------------------------------------------------------------------------
 
 type StandardT m = Fix1T StandardTF m
 
-instance MonadTrans (Fix1T StandardTF) where
+instance (forall m. MonadTrans (t (Fix1T t m))) => MonadTrans (Fix1T t) where
   lift = Fix1T . lift
 
-instance MonadThunkId m => MonadThunkId (Fix1T StandardTF m) where
-  type ThunkId (Fix1T StandardTF m) = ThunkId m
-
 mkStandardT
-  :: ReaderT
-       (Context (StandardT m) (StdValue (StandardT m)))
-       (StateT (HashMap FilePath NExprLoc, Data.HashMap.Strict.HashMap Text Text) m)
-       a
+  :: StandardTFInner (Fix1T StandardTF m) m a
   -> StandardT m a
 mkStandardT = Fix1T . StandardTF
 
 runStandardT
   :: StandardT m a
-  -> ReaderT
-       (Context (StandardT m) (StdValue (StandardT m)))
-       (StateT (HashMap FilePath NExprLoc, Data.HashMap.Strict.HashMap Text Text) m)
-       a
+  -> StandardTFInner (Fix1T StandardTF m) m a
 runStandardT (Fix1T (StandardTF m)) = m
 
 runWithBasicEffects
-  :: (MonadIO m, MonadAtomicRef m) => Options -> StandardT (StdIdT m) a -> m a
+  :: (MonadIO m, MonadAtomicRef m) => Options -> StandardT m a -> m a
 runWithBasicEffects opts =
-  go . (`evalStateT` mempty) . (`runReaderT` newContext opts) . runStandardT
- where
-  go action = do
-    i <- newVar (1 :: Int)
-    runFreshIdT i action
+  (`evalStateT` mempty) . (`runReaderT` newContext opts) . (`runThunkT` nil) . (`runScopeT` mempty) . runStandardT
 
-runWithBasicEffectsIO :: Options -> StandardT (StdIdT IO) a -> IO a
+runWithBasicEffectsIO :: Options -> StandardT IO a -> IO a
 runWithBasicEffectsIO = runWithBasicEffects
diff --git a/src/Nix/String/Coerce.hs b/src/Nix/String/Coerce.hs
index 1c7312d54..6687eea71 100644
--- a/src/Nix/String/Coerce.hs
+++ b/src/Nix/String/Coerce.hs
@@ -42,13 +42,13 @@ coerceToString
   :: ( Framed e m
      , MonadStore m
      , MonadThrow m
-     , MonadDataErrorContext t f m
-     , MonadValue (NValue t f m) m
+     , MonadDataErrorContext f m
+     , MonadValue (NValue f m) m
      )
-  => (NValue t f m -> NValue t f m -> m (NValue t f m))
+  => (NValue f m -> NValue f m -> m (NValue f m))
   -> CopyToStoreMode
   -> CoercionLevel
-  -> NValue t f m
+  -> NValue f m
   -> m NixString
 coerceToString call ctsm clevel = go
  where
diff --git a/src/Nix/Thunk.hs b/src/Nix/Thunk.hs
index 08e0c8f57..cecb12cd4 100644
--- a/src/Nix/Thunk.hs
+++ b/src/Nix/Thunk.hs
@@ -1,60 +1,75 @@
-{-# LANGUAGE DefaultSignatures #-}
+{-# LANGUAGE StandaloneDeriving #-}
+{-# LANGUAGE ApplicativeDo #-}
+{-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE FlexibleContexts #-}
-{-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE QuantifiedConstraints #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
 
 module Nix.Thunk where
 
 import           Control.Exception              ( Exception )
-import           Control.Monad.Trans.Class      ( MonadTrans(..) )
-import           Control.Monad.Trans.Except
+import           Control.Monad.Except
 import           Control.Monad.Trans.Reader
 import           Control.Monad.Trans.State
 import           Control.Monad.Trans.Writer
 import           Data.Typeable                  ( Typeable )
+import           Nix.Utils.Fix1
+
+class MonadTransWrap t where
+  --TODO: Can we enforce that the resulting function is as linear as the provided one?
+  liftWrap :: Monad m => (forall x. m x -> m x) -> t m a -> t m a
+
+instance MonadTransWrap (ReaderT s) where
+  liftWrap f a = do
+    env <- ask
+    lift $ f $ runReaderT a env
+
+instance Monoid w => MonadTransWrap (WriterT w) where
+  liftWrap f a = do
+    (result, w) <- lift $ f $ runWriterT a
+    tell w
+    pure result
+
+instance MonadTransWrap (ExceptT e) where
+  liftWrap f a = do
+    lift (f $ runExceptT a) >>= \case
+      Left e -> throwError e
+      Right result -> pure result
+
+instance MonadTransWrap (StateT s) where
+  liftWrap f a = do
+    old <- get
+    (result, new) <- lift $ f $ runStateT a old
+    put new
+    pure result
+
+instance (forall m. MonadTransWrap (t (Fix1T t m))) => MonadTransWrap (Fix1T t) where
+  liftWrap f (Fix1T a) = Fix1T $ liftWrap f a
+
 
 class ( Monad m
-      , Eq (ThunkId m)
-      , Ord (ThunkId m)
-      , Show (ThunkId m)
-      , Typeable (ThunkId m)
-      )
-      => MonadThunkId m where
-  type ThunkId m :: *
-  freshId :: m (ThunkId m)
-  default freshId
-      :: ( MonadThunkId m'
-        , MonadTrans t
-        , m ~ t m'
-        , ThunkId m ~ ThunkId m'
-        )
-      => m (ThunkId m)
-  freshId = lift freshId
-
-instance MonadThunkId m => MonadThunkId (ReaderT r m) where
-  type ThunkId (ReaderT r m) = ThunkId m
-instance (Monoid w, MonadThunkId m) => MonadThunkId (WriterT w m) where
-  type ThunkId (WriterT w m) = ThunkId m
-instance MonadThunkId m => MonadThunkId (ExceptT e m) where
-  type ThunkId (ExceptT e m) = ThunkId m
-instance MonadThunkId m => MonadThunkId (StateT s m) where
-  type ThunkId (StateT s m) = ThunkId m
-
-class MonadThunkId m => MonadThunk t m a | t -> m, t -> a where
-  thunk :: m a -> m t
-
-  -- | Return an identifier for the thunk unless it is a pure value (i.e.,
-  --   strictly an encapsulation of some 'a' without any additional
-  --   structure). For pure values represented as thunks, returns mempty.
-  thunkId :: t -> ThunkId m
-
-  queryM :: t -> m r -> (a -> m r) -> m r
-  force :: t -> (a -> m r) -> m r
-  forceEff :: t -> (a -> m r) -> m r
+      , Eq (Thunk m)
+      , Ord (Thunk m)
+      , Show (Thunk m)
+      , Typeable (Thunk m)
+      ) => MonadThunk m where
+  type Thunk m :: *
+  type ThunkValue m :: *
+  thunk :: m (ThunkValue m) -> m (Thunk m)
+
+  queryM :: Thunk m -> m r -> (ThunkValue m -> m r) -> m r
+  force :: Thunk m -> (ThunkValue m -> m r) -> m r
+  forceEff :: Thunk m -> (ThunkValue m -> m r) -> m r
 
   -- | Modify the action to be performed by the thunk. For some implicits
   --   this modifies the thunk, for others it may create a new thunk.
-  further :: t -> (m a -> m a) -> m t
+  further :: Thunk m -> (m (ThunkValue m) -> m (ThunkValue m)) -> m (Thunk m)
+
+deriving instance MonadThunk (t (Fix1T t m) m) => MonadThunk (Fix1T t m)
 
 newtype ThunkLoop = ThunkLoop String -- contains rendering of ThunkId
   deriving Typeable
diff --git a/src/Nix/Thunk/Basic.hs b/src/Nix/Thunk/Basic.hs
index 5e25b17b1..17aaefedb 100644
--- a/src/Nix/Thunk/Basic.hs
+++ b/src/Nix/Thunk/Basic.hs
@@ -1,52 +1,107 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE DeriveTraversable #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TupleSections #-}
 {-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE StandaloneDeriving #-}
 
 
 
-module Nix.Thunk.Basic (NThunkF(..), Deferred(..), MonadBasicThunk) where
+module Nix.Thunk.Basic (ThunkT (..), runThunkT, NThunkF (..), Deferred (..)) where
 
 import           Control.Exception       hiding ( catch )
 import           Control.Monad.Catch
+import           Control.Monad.Reader
+import           Control.Monad.State
 
+import           Nix.Effects
 import           Nix.Thunk
 import           Nix.Var
+import           Nix.Thunk.StableId
+import           Nix.Fresh.Stable
+import           Control.Applicative
+import           Control.Monad.Ref
+import Data.Typeable
+#ifdef MIN_VERSION_haskeline
+import System.Console.Haskeline.MonadException hiding(catch)
+#endif
+
+newtype ThunkT v m a = ThunkT { unThunkT :: FreshStableIdT m a }
+  deriving
+    ( Functor
+    , Applicative
+    , Alternative
+    , Monad
+    , MonadPlus
+    , MonadFail
+    , MonadFix
+    , MonadRef
+    , MonadAtomicRef
+    , MonadIO
+    , MonadCatch
+    , MonadThrow
+#ifdef MIN_VERSION_haskeline
+    , MonadException
+#endif
+    , MonadMask
+    , MonadStore
+    )
+
+deriving instance MonadState s m => MonadState s (ThunkT v m)
+
+instance MonadReader r m => MonadReader r (ThunkT v m) where
+  ask = lift ask
+  local f = liftWrap $ local f
+  reader = lift . reader
+
+runThunkT :: Monad m => ThunkT v m a -> StableId -> m a
+runThunkT (ThunkT a) root = runFreshStableIdT root a
+
+instance MonadTrans (ThunkT v) where
+  lift = ThunkT . lift
+
+instance MonadTransWrap (ThunkT v) where
+  liftWrap f (ThunkT a) = ThunkT $ liftWrap f a
 
 data Deferred m v = Deferred (m v) | Computed v
     deriving (Functor, Foldable, Traversable)
 
 -- | The type of very basic thunks
 data NThunkF m v
-    = Thunk (ThunkId m) (Var m Bool) (Var m (Deferred m v))
+    = Thunk StableId (Var m Bool) (Var m (Deferred m v))
 
-instance (Eq v, Eq (ThunkId m)) => Eq (NThunkF m v) where
+instance Eq (NThunkF m v) where
   Thunk x _ _ == Thunk y _ _ = x == y
 
-instance Show v => Show (NThunkF m v) where
-  show Thunk{} = "<thunk>"
+instance Ord (NThunkF m v) where
+  Thunk x _ _ `compare` Thunk y _ _ = x `compare` y
 
-type MonadBasicThunk m = (MonadThunkId m, MonadVar m)
+instance Show (NThunkF m v) where
+  show (Thunk tid _ _) = "<thunk " <> show tid <> ">"
 
-instance (MonadBasicThunk m, MonadCatch m)
-  => MonadThunk (NThunkF m v) m v where
+instance (Typeable v, Typeable m, MonadAtomicRef m, MonadCatch m)
+  => MonadThunk (ThunkT v m) where
+  type Thunk (ThunkT v m) = NThunkF m v
+  type ThunkValue (ThunkT v m) = v
   thunk = buildThunk
-  thunkId (Thunk n _ _) = n
-  queryM   = queryThunk
-  force    = forceThunk
+  queryM = queryThunk
+  force = forceThunk
   forceEff = forceEffects
-  further  = furtherThunk
+  further t f = thunk $ f $ force t pure
 
-buildThunk :: MonadBasicThunk m => m v -> m (NThunkF m v)
-buildThunk action = do
+buildThunk :: MonadRef m => ThunkT v m v -> ThunkT v m (NThunkF m v)
+buildThunk (ThunkT action) = ThunkT $ do
   freshThunkId <- freshId
-  Thunk freshThunkId <$> newVar False <*> newVar (Deferred action)
+  Thunk freshThunkId <$> newVar False <*> newVar (Deferred $ runFreshStableIdT freshThunkId action)
 
-queryThunk :: MonadVar m => NThunkF m v -> m a -> (v -> m a) -> m a
+queryThunk :: MonadVar m => NThunkF m v -> ThunkT v m a -> (v -> ThunkT v m a) -> ThunkT v m a
 queryThunk (Thunk _ active ref) n k = do
   nowActive <- atomicModifyVar active (True, )
   if nowActive
@@ -61,10 +116,10 @@ queryThunk (Thunk _ active ref) n k = do
 
 forceThunk
   :: forall m v a
-   . (MonadVar m, MonadThrow m, MonadCatch m, Show (ThunkId m))
+   . (MonadVar m, MonadThrow m, MonadCatch m, Show StableId)
   => NThunkF m v
-  -> (v -> m a)
-  -> m a
+  -> (v -> ThunkT v m a)
+  -> ThunkT v m a
 forceThunk (Thunk n active ref) k = do
   eres <- readVar ref
   case eres of
@@ -74,14 +129,14 @@ forceThunk (Thunk n active ref) k = do
       if nowActive
         then throwM $ ThunkLoop $ show n
         else do
-          v <- catch action $ \(e :: SomeException) -> do
+          v <- catch (ThunkT $ lift action) $ \(e :: SomeException) -> do
             _ <- atomicModifyVar active (False, )
             throwM e
           _ <- atomicModifyVar active (False, )
           writeVar ref (Computed v)
           k v
 
-forceEffects :: MonadVar m => NThunkF m v -> (v -> m r) -> m r
+forceEffects :: MonadVar m => NThunkF m v -> (v -> ThunkT v m r) -> ThunkT v m r
 forceEffects (Thunk _ active ref) k = do
   nowActive <- atomicModifyVar active (True, )
   if nowActive
@@ -91,14 +146,17 @@ forceEffects (Thunk _ active ref) k = do
       case eres of
         Computed v      -> k v
         Deferred action -> do
-          v <- action
+          v <- ThunkT $ lift action
           writeVar ref (Computed v)
           _ <- atomicModifyVar active (False, )
           k v
 
+{-
+--[ryantrinkle] I'm worried about what impact this will have on the way withRootId works
 furtherThunk :: MonadVar m => NThunkF m v -> (m v -> m v) -> m (NThunkF m v)
 furtherThunk t@(Thunk _ _ ref) k = do
   _ <- atomicModifyVar ref $ \x -> case x of
     Computed _ -> (x, x)
     Deferred d -> (Deferred (k d), x)
   pure t
+-}
diff --git a/src/Nix/Thunk/Separate.hs b/src/Nix/Thunk/Separate.hs
new file mode 100644
index 000000000..caa305c84
--- /dev/null
+++ b/src/Nix/Thunk/Separate.hs
@@ -0,0 +1,162 @@
+{-# LANGUAGE BangPatterns #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE ConstraintKinds #-}
+{-# LANGUAGE DeriveFoldable #-}
+{-# LANGUAGE DeriveFunctor #-}
+{-# LANGUAGE DeriveTraversable #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TupleSections #-}
+{-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE UndecidableInstances #-}
+
+{-# OPTIONS_GHC -Wno-orphans #-}
+
+module Nix.Thunk.Separate (NThunkF(..), MonadSeparateThunk, runSeparateThunkT, askThunkCache) where
+
+import Control.Exception hiding (catch)
+import Control.Monad.Catch
+import Control.Monad.Reader
+import Control.Monad.Ref
+import Data.Map.Strict (Map)
+import qualified Data.Map.Strict as Map
+
+import Nix.Thunk
+
+-- | The type of very basic thunks
+data NThunkF m v
+    = Value v
+    | Thunk (ThunkId m) (SeparateThunkT v m v)
+
+instance (Eq v, Eq (ThunkId m)) => Eq (NThunkF m v) where
+    Value x == Value y = x == y
+    Thunk x _ == Thunk y _ = x == y
+    _ == _ = False              -- jww (2019-03-16): not accurate...
+
+instance Show v => Show (NThunkF m v) where
+    show (Value v) = show v
+    show (Thunk _ _) = "<thunk>"
+
+type MonadSeparateThunk m = (MonadThunkId m, MonadAtomicRef m, Ord (ThunkId m)) --TODO: ThunkId allocation also needs to be sufficiently deterministic
+
+type ThunkCache m v = Ref m (Map (ThunkId m) (Maybe v))
+
+--TODO: HashMap?
+newtype SeparateThunkT v m a = SeparateThunkT (ReaderT (ThunkCache m v) m a)
+  deriving
+    ( Functor
+    , Applicative
+    , Monad
+    , MonadRef
+    , MonadAtomicRef
+    , MonadCatch
+    , MonadThrow
+    )
+
+askThunkCache :: Monad m => SeparateThunkT v m (ThunkCache m v)
+askThunkCache = SeparateThunkT ask
+
+runSeparateThunkT :: ThunkCache m v -> SeparateThunkT v m a -> m a
+runSeparateThunkT c (SeparateThunkT a) = runReaderT a c
+
+instance MonadTrans (SeparateThunkT v) where
+    lift = SeparateThunkT . lift
+
+instance MonadThunkId m => MonadThunkId (SeparateThunkT v m) where
+    type ThunkId (SeparateThunkT v m) = ThunkId m
+
+instance (MonadSeparateThunk m, MonadCatch m)
+  => MonadThunk (NThunkF m v) (SeparateThunkT v m) v where
+    thunk     = buildThunk
+    thunkId   = \case
+        Value _   -> Nothing
+        Thunk n _ -> Just n
+    query     = queryValue
+    queryM    = queryThunk
+    force     = forceThunk
+    forceEff  = forceEffects
+    wrapValue = valueRef
+    getValue  = thunkValue
+
+valueRef :: v -> NThunkF m v
+valueRef = Value
+
+thunkValue :: NThunkF m v -> Maybe v
+thunkValue (Value v) = Just v
+thunkValue _ = Nothing
+
+buildThunk :: MonadThunkId m => SeparateThunkT v m v -> SeparateThunkT v m (NThunkF m v)
+buildThunk action = do
+    freshThunkId <- lift freshId
+    return $ Thunk freshThunkId action
+
+queryValue :: NThunkF m v -> a -> (v -> a) -> a
+queryValue (Value v) _ k = k v
+queryValue _ n _ = n
+
+queryThunk :: (MonadAtomicRef m, Ord (ThunkId m)) => NThunkF m v -> SeparateThunkT v m a -> (v -> SeparateThunkT v m a) -> SeparateThunkT v m a
+queryThunk (Value v) _ k = k v
+queryThunk (Thunk tid _) n k = do
+    c <- SeparateThunkT ask
+    mOldVal <- atomicModifyRef' c $ \old ->
+        -- Try to insert Nothing into the given key, but if something is already
+        -- there, just leave it
+        let (mOldVal, !new) = Map.insertLookupWithKey (\_ _ oldVal -> oldVal) tid Nothing old
+        in (new, mOldVal)
+    case mOldVal of
+        Nothing -> do
+            result <- n -- Not computed, inactive
+            -- This is the only case where we've actually changed c, so restore it
+            atomicModifyRef' c $ \old -> (Map.delete tid old, ())
+            return result
+        Just Nothing -> n -- Active
+        Just (Just v) -> k v -- Computed, inactive
+
+forceThunk
+    :: forall m v a.
+    ( MonadAtomicRef m
+    , MonadThrow m
+    , MonadCatch m
+    , Show (ThunkId m)
+    , Ord (ThunkId m)
+    )
+    => NThunkF m v -> (v -> SeparateThunkT v m a) -> SeparateThunkT v m a
+forceThunk (Value v) k = k v
+forceThunk (Thunk tid action) k = do
+    c <- SeparateThunkT ask
+    mOldVal <- atomicModifyRef' c $ \old ->
+        -- Try to insert Nothing into the given key, but if something is already
+        -- there, just leave it
+        let (mOldVal, !new) = Map.insertLookupWithKey (\_ _ oldVal -> oldVal) tid Nothing old
+        in (new, mOldVal)
+    case mOldVal of
+        Nothing -> do -- Not computed, inactive
+            v <- catch action $ \(e :: SomeException) -> do
+                -- This is the only case where we've actually changed c, so restore it
+                _ <- atomicModifyRef' c $ \old -> (Map.delete tid old, ())
+                throwM e
+            atomicModifyRef' c $ \old -> (Map.insert tid (Just v) old, ())
+            k v
+        Just Nothing -> throwM $ ThunkLoop $ show tid
+        Just (Just v) -> k v -- Computed, inactive
+
+forceEffects :: (MonadAtomicRef m, Ord (ThunkId m)) => NThunkF m v -> (v -> SeparateThunkT v m r) -> SeparateThunkT v m r
+forceEffects (Value v) k = k v
+forceEffects (Thunk tid action) k = do
+    c <- SeparateThunkT ask
+    mOldVal <- atomicModifyRef' c $ \old ->
+        -- Try to insert Nothing into the given key, but if something is already
+        -- there, just leave it
+        let (mOldVal, !new) = Map.insertLookupWithKey (\_ _ oldVal -> oldVal) tid Nothing old
+        in (new, mOldVal)
+    case mOldVal of
+        Nothing -> do -- Not computed, inactive
+            v <- action
+            atomicModifyRef' c $ \old -> (Map.insert tid (Just v) old, ())
+            k v
+        Just Nothing -> return $ error "Loop detected"
+        Just (Just v) -> k v -- Computed, inactive
diff --git a/src/Nix/Thunk/StableId.hs b/src/Nix/Thunk/StableId.hs
new file mode 100644
index 000000000..df107924f
--- /dev/null
+++ b/src/Nix/Thunk/StableId.hs
@@ -0,0 +1,81 @@
+{-# LANGUAGE MultiWayIf #-}
+{-# LANGUAGE MagicHash #-}
+
+-- Equivalent to [Int], but with near-O(1) amortized comparison
+module Nix.Thunk.StableId (StableId, nil, cons, uncons) where
+
+import Data.IORef
+import System.IO.Unsafe
+import GHC.Prim
+import Data.Hashable
+import Data.List (unfoldr)
+import Data.Ord
+
+--TODO: If we have a really long chain, we will keep leaking memory; what can we do about this?
+
+data StableId = StableId
+  { _stableId_value :: {-# UNPACK #-} !Int
+  , _stableId_hash :: {-# UNPACK #-} !Int
+  , _stableId_parent :: {-# UNPACK #-} !(IORef StableId)
+  }
+
+{-# NOINLINE nil #-} -- If nil is not a single value on the heap, infinite recursion can result
+nil :: StableId
+nil = StableId 0 0 $ unsafePerformIO $ newIORef $ error "nil"
+
+cons :: Int -> StableId -> StableId
+cons v p@(StableId _ ph _) = StableId v (hash (v, ph)) $ unsafeDupablePerformIO $ newIORef p
+
+uncons :: StableId -> Maybe (Int, StableId)
+uncons s = if _stableId_parent s == _stableId_parent nil
+  then Nothing
+  else Just
+  ( _stableId_value s
+  , unsafeDupablePerformIO $ readIORef $ _stableId_parent s
+  )
+
+--TODO: Reimplement Eq in terms of Ord?
+instance Eq StableId where
+  a == b = if
+    | _stableId_parent a == _stableId_parent b -- We're the exact same heap object
+      -> True
+    | _stableId_hash a /= _stableId_hash b || _stableId_value a /= _stableId_value b -- We're definitely different
+      -> False
+    | otherwise -- Different objects, but same value and hash.  These are either the same value or a hash collision.
+      -> unsafeDupablePerformIO $ do
+           pa <- readIORef $ _stableId_parent a
+           pb <- readIORef $ _stableId_parent b
+           case reallyUnsafePtrEquality# pa pb of
+             -- Parents are different objects
+             0# -> if pa == pb
+               then do writeIORef (_stableId_parent b) pa -- Parents are equivalent, so unify
+                       return True
+               else return False -- Parents are not equivalent, so leave them alone
+             -- Parents are the same object already
+             _ -> return True
+
+instance Ord StableId where
+  a `compare` b = case comparing _stableId_hash a b <> comparing _stableId_value a b of
+    LT -> LT
+    GT -> GT
+    EQ -> case _stableId_parent a == _stableId_parent b of
+      True -> EQ
+      False -> unsafeDupablePerformIO $ do
+        pa <- readIORef $ _stableId_parent a
+        pb <- readIORef $ _stableId_parent b
+        case reallyUnsafePtrEquality# pa pb of
+          -- Parents are different objects
+          0# -> case pa `compare` pb of
+            LT -> return LT
+            GT -> return GT
+            EQ -> do
+              writeIORef (_stableId_parent b) pa
+              return EQ
+          -- Parents are the same object already
+          _ -> return EQ
+
+toList :: StableId -> [Int]
+toList = unfoldr uncons
+
+instance Show StableId where
+  showsPrec n = showsPrec n . toList
diff --git a/src/Nix/Type/Infer.hs b/src/Nix/Type/Infer.hs
index 88a729811..ecff86599 100644
--- a/src/Nix/Type/Infer.hs
+++ b/src/Nix/Type/Infer.hs
@@ -58,9 +58,11 @@ import           Nix.Eval                       ( MonadEval(..) )
 import qualified Nix.Eval                      as Eval
 import           Nix.Expr.Types
 import           Nix.Expr.Types.Annotated
-import           Nix.Fresh
+import           Nix.Fresh ()
 import           Nix.String
 import           Nix.Scope
+import           Nix.Thunk.StableId
+import           Nix.Fresh.Stable
 import qualified Nix.Type.Assumption           as As
 import           Nix.Type.Env
 import qualified Nix.Type.Env                  as Env
@@ -222,10 +224,9 @@ runInfer' =
     . (`runReaderT` (Set.empty, emptyScopes))
     . getInfer
 
-runInfer :: (forall s . InferT s (FreshIdT Int (ST s)) a) -> Either InferError a
+runInfer :: (forall s. InferT s (FreshStableIdT (ST s)) a) -> Either InferError a
 runInfer m = runST $ do
-  i <- newVar (1 :: Int)
-  runFreshIdT i (runInfer' m)
+  runFreshStableIdT nil (runInfer' m)
 
 inferType
   :: forall s m . MonadInfer m => Env -> NExpr -> InferT s m [(Subst, Type)]
@@ -694,8 +695,8 @@ solve cs = solve' (nextSolvable cs)
     s' <- lift $ instantiate s
     solve (EqConst t s' : cs)
 
-instance Monad m => Scoped (Judgment s) (InferT s m) where
+instance Monad m => Scoped (InferT s m) (Judgment s) (InferT s m) where
   currentScopes = currentScopesReader
   clearScopes   = clearScopesReader @(InferT s m) @(Judgment s)
   pushScopes    = pushScopesReader
-  lookupVar     = lookupVarReader
+  askLookupVar  = lookupVarReader
diff --git a/src/Nix/Utils/Fix1.hs b/src/Nix/Utils/Fix1.hs
index 2595fefb6..4f853e66b 100644
--- a/src/Nix/Utils/Fix1.hs
+++ b/src/Nix/Utils/Fix1.hs
@@ -29,6 +29,7 @@ import           Control.Monad.Catch            ( MonadCatch
 import           Control.Monad.Reader           ( MonadReader )
 import           Control.Monad.State            ( MonadState )
 
+
 -- | The fixpoint combinator, courtesy of Gregory Malecha.
 --   https://gist.github.com/gmalecha/ceb3778b9fdaa4374976e325ac8feced
 newtype Fix1 (t :: (k -> *) -> k -> *) (a :: k) = Fix1 { unFix1 :: t (Fix1 t) a }
@@ -64,7 +65,6 @@ deriving instance MonadMask (t (Fix1T t m) m) => MonadMask (Fix1T t m)
 deriving instance MonadReader e (t (Fix1T t m) m) => MonadReader e (Fix1T t m)
 deriving instance MonadState s (t (Fix1T t m) m) => MonadState s (Fix1T t m)
 
-
 type MonadFix1T t m = (MonadTrans (Fix1T t), Monad (t (Fix1T t m) m))
 
 instance (MonadFix1T t m, MonadRef m) => MonadRef (Fix1T t m) where
@@ -73,10 +73,8 @@ instance (MonadFix1T t m, MonadRef m) => MonadRef (Fix1T t m) where
   readRef = lift . readRef
   writeRef r = lift . writeRef r
 
-
 instance (MonadFix1T t m, MonadAtomicRef m) => MonadAtomicRef (Fix1T t m) where
   atomicModifyRef r = lift . atomicModifyRef r
-
 {-
 
 newtype Flip (f :: i -> j -> *) (a :: j) (b :: i) = Flip { unFlip :: f b a }
diff --git a/src/Nix/Value.hs b/src/Nix/Value.hs
index f7bbb4f4f..64e41c502 100644
--- a/src/Nix/Value.hs
+++ b/src/Nix/Value.hs
@@ -5,6 +5,7 @@
 {-# LANGUAGE DeriveTraversable #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE GADTs #-}
 {-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE OverloadedStrings #-}  -- @key@ in Aeson reports: @key@ often has better inference than @ix@ when used with OverloadedStrings.
 {-# LANGUAGE PatternSynonyms #-}
@@ -12,6 +13,7 @@
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE UndecidableInstances #-}
 {-# LANGUAGE ViewPatterns #-}
 
 {-# OPTIONS_GHC -Wno-missing-signatures #-}
@@ -202,21 +204,21 @@ hoistNValueF lft = \case
 
 -- | At the time of constructor, the expected arguments to closures are values
 --   that may contain thunks. The type of such thunks are fixed at that time.
-newtype NValue' t f m a =
+newtype NValue' f m a =
   NValue
     {
     -- | Applying F-algebra carrier (@NValue@) to the F-algebra Base functor data type (@NValueF@), forming the \( F(A)-> A \)).
-    _nValue :: f (NValueF (NValue t f m) m a)
+    _nValue :: f (NValueF (NValue f m) m a)
     }
   deriving (Generic, Typeable, Functor, Foldable, Eq1)
 
-instance (Comonad f, Show a) => Show (NValue' t f m a) where
+instance (Comonad f, Show a) => Show (NValue' f m a) where
   show (NValue (extract -> v)) = show v
 
 
 -- ** Show1
 
-instance Comonad f => Show1 (NValue' t f m) where
+instance Comonad f => Show1 (NValue' f m) where
   liftShowsPrec sp sl p = \case
     NVConstant' atom  -> showsUnaryWith showsPrec "NVConstantF" p atom
     NVStr' ns ->
@@ -235,8 +237,8 @@ instance Comonad f => Show1 (NValue' t f m) where
 sequenceNValue'
   :: (Functor n, Traversable f, Monad m, Applicative n)
   => (forall x . n x -> m x)
-  -> NValue' t f m (n a)
-  -> n (NValue' t f m a)
+  -> NValue' f m (n a)
+  -> n (NValue' f m a)
 sequenceNValue' transform (NValue v) =
   NValue <$> traverse (sequenceNValueF transform) v
 
@@ -259,22 +261,22 @@ lmapNValueF f = \case
 
 -- | @iter@
 iterNValue'
-  :: forall t f m a r
+  :: forall f m a r
    . MonadDataContext f m
-  => (a -> (NValue' t f m a -> r) -> r)
-  -> (NValue' t f m r -> r)
-  -> NValue' t f m a
+  => (a -> (NValue' f m a -> r) -> r)
+  -> (NValue' f m r -> r)
+  -> NValue' f m a
   -> r
 iterNValue' k f = f . fmap (\a -> k a (iterNValue' k f))
 
 
 -- | @hoistFree@: Back & forth hoisting in the monad stack
 hoistNValue'
-  :: (Functor m, Functor n, Functor f)
+  :: (Functor m, Functor n, Functor f, Thunk m ~ Thunk n)
   => (forall x . n x -> m x)
   -> (forall x . m x -> n x)
-  -> NValue' t f m a
-  -> NValue' t f n a
+  -> NValue' f m a
+  -> NValue' f n a
 hoistNValue' run lft (NValue v) =
     NValue $ lmapNValueF (hoistNValue lft run) . hoistNValueF lft <$> v
 
@@ -286,8 +288,8 @@ bindNValue'
   :: (Traversable f, Monad m, Monad n)
   => (forall x . n x -> m x)
   -> (a -> n b)
-  -> NValue' t f m a
-  -> n (NValue' t f m b)
+  -> NValue' f m a
+  -> n (NValue' f m b)
 bindNValue' transform f (NValue v) =
   NValue <$> traverse (bindNValueF transform f) v
 
@@ -295,20 +297,20 @@ bindNValue' transform f (NValue v) =
 
 -- | @lift@
 liftNValue'
-  :: (MonadTrans u, Monad m, Functor (u m), Functor f)
+  :: (MonadTrans u, Monad m, Functor (u m), Functor f, Thunk m ~ Thunk (u m))
   => (forall x . u m x -> m x)
-  -> NValue' t f m a
-  -> NValue' t f (u m) a
+  -> NValue' f m a
+  -> NValue' f (u m) a
 liftNValue' run = hoistNValue' run lift
 
 -- **** MonadTransUnlift
 
 -- | @unlift@
 unliftNValue'
-  :: (MonadTrans u, Monad m, Functor (u m), Functor f)
+  :: (MonadTrans u, Monad m, Functor (u m), Functor f, Thunk m ~ Thunk (u m))
   => (forall x . u m x -> m x) -- aka "run"
-  -> NValue' t f (u m) a
-  -> NValue' t f m a
+  -> NValue' f (u m) a
+  -> NValue' f m a
 unliftNValue' = hoistNValue' lift
 
 
@@ -337,28 +339,26 @@ unliftNValue' = hoistNValue' lift
 -- | Haskell constant to the Nix constant,
 nvConstant' :: Applicative f
   => NAtom
-  -> NValue' t f m r
+  -> NValue' f m r
 nvConstant' = NValue . pure . NVConstantF
 
-
 -- | Haskell text & context to the Nix text & context,
 nvStr' :: Applicative f
   => NixString
-  -> NValue' t f m r
+  -> NValue' f m r
 nvStr' = NValue . pure . NVStrF
 
-
 -- | Haskell @FilePath@ to the Nix path,
 nvPath' :: Applicative f
   => FilePath
-  -> NValue' t f m r
+  -> NValue' f m r
 nvPath' = NValue . pure . NVPathF
 
 
 -- | Haskell @[]@ to the Nix @[]@,
 nvList' :: Applicative f
   => [r]
-  -> NValue' t f m r
+  -> NValue' f m r
 nvList' = NValue . pure . NVListF
 
 
@@ -366,25 +366,25 @@ nvList' = NValue . pure . NVListF
 nvSet' :: Applicative f
   => HashMap Text r
   -> HashMap Text SourcePos
-  -> NValue' t f m r
+  -> NValue' f m r
 nvSet' s x = NValue $ pure $ NVSetF s x
 
 
 -- | Haskell closure to the Nix closure,
 nvClosure' :: (Applicative f, Functor m)
   => Params ()
-  -> (NValue t f m
+  -> (NValue f m
       -> m r
     )
-  -> NValue' t f m r
+  -> NValue' f m r
 nvClosure' x f = NValue $ pure $ NVClosureF x f
 
 
 -- | Haskell functions to the Nix functions!
 nvBuiltin' :: (Applicative f, Functor m)
   => String
-  -> (NValue t f m -> m r)
-  -> NValue' t f m r
+  -> (NValue f m -> m r)
+  -> NValue' f m r
 nvBuiltin' name f = NValue $ pure $ NVBuiltinF name f
 
 
@@ -411,12 +411,12 @@ pattern NVBuiltin' name f <- NValue (extract -> NVBuiltinF name f)
 
 -- * @__NValue__@: Nix language values
 
--- | 'NValue t f m' is
+-- | 'NValue f m' is
 --   a value in head normal form (it means only the tip of it has been
 --   evaluated to the normal form, while the rest of it is in lazy
 --   not evaluated form (thunk), this known as WHNF).
 --
---   An action 'm (NValue t f m)' is a pending evaluation that
+--   An action 'm (NValue f m)' is a pending evaluation that
 --   has yet to be performed.
 --
 --   An 't' is either:
@@ -426,18 +426,19 @@ pattern NVBuiltin' name f <- NValue (extract -> NVBuiltinF name f)
 --   The 'Free' structure is used here to represent the possibility that
 --   Nix language allows cycles that may appear during normalization.
 
-type NValue t f m = Free (NValue' t f m) t
+--TODO: What does the `f` represent
+type NValue f m = Free (NValue' f m) (Thunk m)
 
 
 -- ** Free
 
 -- | @iter@
 iterNValue
-  :: forall t f m r
+  :: forall f m r
    . MonadDataContext f m
-  => (t -> (NValue t f m -> r) -> r)
-  -> (NValue' t f m r -> r)
-  -> NValue t f m
+  => (Thunk m -> (NValue f m -> r) -> r)
+  -> (NValue' f m r -> r)
+  -> NValue f m
   -> r
 iterNValue k f = iter f . fmap (\t -> k t (iterNValue k f))
 
@@ -446,9 +447,9 @@ iterNValue k f = iter f . fmap (\t -> k t (iterNValue k f))
 iterNValueM
   :: (MonadDataContext f m, Monad n)
   => (forall x . n x -> m x)
-  -> (t -> (NValue t f m -> n r) -> n r)
-  -> (NValue' t f m (n r) -> n r)
-  -> NValue t f m
+  -> (Thunk m -> (NValue f m -> n r) -> n r)
+  -> (NValue' f m (n r) -> n r)
+  -> NValue f m
   -> n r
 iterNValueM transform k f =
     iterM f <=< go . fmap (\t -> k t (iterNValueM transform k f))
@@ -459,35 +460,34 @@ iterNValueM transform k f =
 
 -- | @hoistFree@, Back & forth hoisting in the monad stack
 hoistNValue
-  :: (Functor m, Functor n, Functor f)
+  :: (Functor m, Functor n, Functor f, Thunk m ~ Thunk n)
   => (forall x . n x -> m x)
   -> (forall x . m x -> n x)
-  -> NValue t f m
-  -> NValue t f n
+  -> NValue f m
+  -> NValue f n
 hoistNValue run lft = hoistFree (hoistNValue' run lft)
 
-
 -- ** MonadTrans
 
 -- | @lift@
 liftNValue
-  :: (MonadTrans u, Monad m, Functor (u m), Functor f)
+  :: (MonadTrans u, Monad m, Functor (u m), Functor f, Thunk m ~ Thunk (u m))
   => (forall x . u m x -> m x)
-  -> NValue t f m
-  -> NValue t f (u m)
+  -> NValue f m
+  -> NValue f (u m)
 liftNValue run = hoistNValue run lift
 
 
+
 -- *** MonadTransUnlift
 -- | @unlift@
 unliftNValue
-  :: (MonadTrans u, Monad m, Functor (u m), Functor f)
+  :: (MonadTrans u, Monad m, Functor (u m), Functor f, Thunk m ~ Thunk (u m))
   => (forall x . u m x -> m x)  -- aka "run"
-  -> NValue t f (u m)
-  -> NValue t f m
+  -> NValue f (u m)
+  -> NValue f m
 unliftNValue = hoistNValue lift
 
-
 -- ** Methods @F: Hask → NValue@
 --
 -- $Methods @F: Hask → NValue@
@@ -499,8 +499,8 @@ unliftNValue = hoistNValue lift
 -- | Life of a Haskell thunk to the life of a Nix thunk,
 pattern NVThunk t <- Pure t
 nvThunk :: Applicative f
-  => t
-  -> NValue t f m
+  => Thunk m
+  -> NValue f m
 nvThunk = Pure
 
 
@@ -508,7 +508,7 @@ nvThunk = Pure
 pattern NVConstant x <- Free (NVConstant' x)
 nvConstant :: Applicative f
   => NAtom
-  -> NValue t f m
+  -> NValue f m
 nvConstant = Free . nvConstant'
 
 
@@ -516,7 +516,7 @@ nvConstant = Free . nvConstant'
 pattern NVStr ns <- Free (NVStr' ns)
 nvStr :: Applicative f
   => NixString
-  -> NValue t f m
+  -> NValue f m
 nvStr = Free . nvStr'
 
 
@@ -524,75 +524,75 @@ nvStr = Free . nvStr'
 pattern NVPath x <- Free (NVPath' x)
 nvPath :: Applicative f
   => FilePath
-  -> NValue t f m
+  -> NValue f m
 nvPath = Free . nvPath'
 
 
 pattern NVList l <- Free (NVList' l)
 nvList :: Applicative f
-  => [NValue t f m]
-  -> NValue t f m
+  => [NValue f m]
+  -> NValue f m
 nvList = Free . nvList'
 
 
 pattern NVSet s x <- Free (NVSet' s x)
 nvSet :: Applicative f
-  => HashMap Text (NValue t f m)
+  => HashMap Text (NValue f m)
   -> HashMap Text SourcePos
-  -> NValue t f m
+  -> NValue f m
 nvSet s x = Free $ nvSet' s x
 
 
 pattern NVClosure x f <- Free (NVClosure' x f)
 nvClosure :: (Applicative f, Functor m)
   => Params ()
-  -> (NValue t f m
-      -> m (NValue t f m)
+  -> (NValue f m
+      -> m (NValue f m)
     )
-  -> NValue t f m
+  -> NValue f m
 nvClosure x f = Free $ nvClosure' x f
 
 
 pattern NVBuiltin name f <- Free (NVBuiltin' name f)
 nvBuiltin :: (Applicative f, Functor m)
   => String
-  -> (NValue t f m
-    -> m (NValue t f m)
+  -> (NValue f m
+    -> m (NValue f m)
     )
-  -> NValue t f m
+  -> NValue f m
 nvBuiltin name f = Free $ nvBuiltin' name f
 
 
 builtin
-  :: forall m f t
-   . (MonadThunk t m (NValue t f m), MonadDataContext f m)
+  :: forall m f
+   . (MonadThunk m, ThunkValue m ~ NValue f m, MonadDataContext f m)
   => String
-  -> (NValue t f m
-      -> m (NValue t f m)
+  -> (NValue f m
+      -> m (NValue f m)
     )
-  -> m (NValue t f m)
+  -> m (NValue f m)
 builtin name f = pure $ nvBuiltin name $ \a -> f a
 
 
 builtin2
-  :: (MonadThunk t m (NValue t f m), MonadDataContext f m)
+  :: (MonadThunk m, ThunkValue m ~ NValue f m, MonadDataContext f m)
   => String
-  -> (NValue t f m -> NValue t f m
-      -> m (NValue t f m)
+  -> (NValue f m -> NValue f m
+      -> m (NValue f m)
     )
-  -> m (NValue t f m)
+  -> m (NValue f m)
 builtin2 name f = builtin name $ \a -> builtin name $ \b -> f a b
 
 
 builtin3
-  :: (MonadThunk t m (NValue t f m), MonadDataContext f m)
+  :: (MonadThunk m, ThunkValue m ~ NValue f m, MonadDataContext f m)
   => String
-  -> (  NValue t f m
-     -> NValue t f m
-     -> NValue t f m
-     -> m (NValue t f m)
+  -> (  NValue f m
+     -> NValue f m
+     -> NValue f m
+     -> m (NValue f m)
      )
-  -> m (NValue t f m)
+  -> m (NValue f m)
 builtin3 name f =
   builtin name $ \a -> builtin name $ \b -> builtin name $ \c -> f a b c
 
@@ -656,8 +656,8 @@ describeValue = \case
   TBuiltin           -> "a builtin function"
 
 
-showValueType :: (MonadThunk t m (NValue t f m), Comonad f)
-  => NValue t f m
+showValueType :: (MonadThunk m, Thunk m ~ t, ThunkValue m ~ NValue f m, Comonad f)
+  => NValue f m
   -> m String
 showValueType (Pure t) = force t showValueType
 showValueType (Free (NValue (extract -> v))) =
@@ -666,20 +666,20 @@ showValueType (Free (NValue (extract -> v))) =
 
 -- * @ValueFrame@
 
-data ValueFrame t f m
-    = ForcingThunk t
-    | ConcerningValue (NValue t f m)
-    | Comparison (NValue t f m) (NValue t f m)
-    | Addition (NValue t f m) (NValue t f m)
-    | Multiplication (NValue t f m) (NValue t f m)
-    | Division (NValue t f m) (NValue t f m)
+data ValueFrame f m
+    = ForcingThunk (Thunk m)
+    | ConcerningValue (NValue f m)
+    | Comparison (NValue f m) (NValue f m)
+    | Addition (NValue f m) (NValue f m)
+    | Multiplication (NValue f m) (NValue f m)
+    | Division (NValue f m) (NValue f m)
     | Coercion ValueType ValueType
-    | CoercionToJson (NValue t f m)
+    | CoercionToJson (NValue f m)
     | CoercionFromJson Aeson.Value
-    | Expectation ValueType (NValue t f m)
+    | Expectation ValueType (NValue f m)
     deriving Typeable
 
-deriving instance (Comonad f, Show t) => Show (ValueFrame t f m)
+deriving instance (Comonad f, Show (Thunk m)) => Show (ValueFrame f m)
 
 
 -- * @MonadDataContext@
@@ -690,10 +690,10 @@ type MonadDataContext f (m :: * -> *)
 
 -- * @MonadDataErrorContext@
 
-type MonadDataErrorContext t f m
-  = (Show t, Typeable t, Typeable m, Typeable f, MonadDataContext f m)
+type MonadDataErrorContext f m
+  = (Show (Thunk m), Typeable (Thunk m), Typeable m, Typeable f, MonadDataContext f m)
 
-instance MonadDataErrorContext t f m => Exception (ValueFrame t f m)
+instance MonadDataErrorContext f m => Exception (ValueFrame f m)
 
 
 -- ** NValue' traversals, getter & setters
@@ -710,5 +710,5 @@ $(makeLenses ''NValue')
 key
   :: (Traversable f, Applicative g)
   => VarName
-  -> LensLike' g (NValue' t f m a) (Maybe a)
+  -> LensLike' g (NValue' f m a) (Maybe a)
 key k = nValue . traverse . _NVSetF . _1 . hashAt k
diff --git a/src/Nix/Value/Equal.hs b/src/Nix/Value/Equal.hs
index 5c4e8dc16..a81bcb5cd 100644
--- a/src/Nix/Value/Equal.hs
+++ b/src/Nix/Value/Equal.hs
@@ -41,9 +41,9 @@ import           Nix.Utils
 import           Nix.Value
 
 checkComparable
-  :: (Framed e m, MonadDataErrorContext t f m)
-  => NValue t f m
-  -> NValue t f m
+  :: (Framed e m, MonadDataErrorContext f m)
+  => NValue f m
+  -> NValue f m
   -> m ()
 checkComparable x y = case (x, y) of
   (NVConstant (NFloat _), NVConstant (NInt _)) -> pure ()
@@ -145,9 +145,9 @@ compareAttrSets f eq lm rm = runIdentity
 
 valueEqM
   :: forall t f m
-   . (MonadThunk t m (NValue t f m), Comonad f)
-  => NValue t f m
-  -> NValue t f m
+   . (MonadThunk m, Thunk m ~ t, ThunkValue m ~ NValue f m, Comonad f)
+  => NValue f m
+  -> NValue f m
   -> m Bool
 valueEqM (  Pure x) (  Pure y) = thunkEqM x y
 valueEqM (  Pure x) y@(Free _) = thunkEqM x =<< thunk (pure y)
@@ -162,10 +162,10 @@ valueEqM (Free (NValue (extract -> x))) (Free (NValue (extract -> y))) =
     NVStr' s -> pure $ pure s
     _        -> pure mempty
 
-thunkEqM :: (MonadThunk t m (NValue t f m), Comonad f) => t -> t -> m Bool
+thunkEqM :: (MonadThunk m, Thunk m ~ t, ThunkValue m ~ NValue f m, Comonad f) => t -> t -> m Bool
 thunkEqM lt rt = force lt $ \lv -> force rt $ \rv ->
   let unsafePtrEq = case (lt, rt) of
-        (thunkId -> lid, thunkId -> rid) | lid == rid -> pure True
+        (lid, rid) | lid == rid -> pure True
         _ -> valueEqM lv rv
   in  case (lv, rv) of
         (NVClosure _ _, NVClosure _ _) -> unsafePtrEq
diff --git a/src/Nix/XML.hs b/src/Nix/XML.hs
index ba7af1d82..727827f82 100644
--- a/src/Nix/XML.hs
+++ b/src/Nix/XML.hs
@@ -15,7 +15,7 @@ import           Nix.String
 import           Nix.Value
 import           Text.XML.Light
 
-toXML :: forall t f m . MonadDataContext f m => NValue t f m -> NixString
+toXML :: forall f m . MonadDataContext f m => NValue f m -> NixString
 toXML = runWithStringContext . fmap pp . iterNValue (\_ _ -> cyc) phi
  where
   cyc = pure $ mkElem "string" "value" "<expr>"
@@ -27,7 +27,7 @@ toXML = runWithStringContext . fmap pp . iterNValue (\_ _ -> cyc) phi
       . ppElement
       . (\e -> Element (unqual "expr") mempty [Elem e] Nothing)
 
-  phi :: NValue' t f m (WithStringContext Element) -> WithStringContext Element
+  phi :: NValue' f m (WithStringContext Element) -> WithStringContext Element
   phi = \case
     NVConstant' a -> case a of
       NURI   t -> pure $ mkElem "string" "value" (Text.unpack t)
diff --git a/tests/NixLanguageTests.hs b/tests/NixLanguageTests.hs
index 952fcab74..ba07752a8 100644
--- a/tests/NixLanguageTests.hs
+++ b/tests/NixLanguageTests.hs
@@ -21,7 +21,7 @@ import qualified Data.Text                     as Text
 import qualified Data.Text.IO                  as Text
 import           Data.Time
 import           GHC.Exts
-import           Nix.Lint
+--import           Nix.Lint
 import           Nix.Options
 import           Nix.Options.Parser
 import           Nix.Parser
@@ -115,7 +115,7 @@ assertParseFail opts file = do
   catch
       (case eres of
         Success expr -> do
-          _ <- pure $! runST $ void $ lint opts expr
+        --  _ <- pure $! runST $ void $ lint opts expr
           assertFailure
             $  "Unexpected success parsing `"
             <> file
diff --git a/tests/TestCommon.hs b/tests/TestCommon.hs
index 17d2db24a..315325002 100644
--- a/tests/TestCommon.hs
+++ b/tests/TestCommon.hs
@@ -7,14 +7,17 @@ module TestCommon where
 
 import           Control.Monad.Catch
 import           Control.Monad.IO.Class
+import           Data.Functor.Identity
 import           Data.Text                      ( Text
                                                 , unpack
                                                 )
 import           Data.Time
 import           Nix
+import           Nix.Cited
+import           Nix.Context
 import           Nix.Exec                       ( )
 import           Nix.Standard
-import           Nix.Fresh.Basic
+import           Nix.Fresh.Stable
 import           System.Environment
 import           System.IO
 import           System.Posix.Files
@@ -22,7 +25,7 @@ import           System.Posix.Temp
 import           System.Process
 import           Test.Tasty.HUnit
 
-hnixEvalFile :: Options -> FilePath -> IO (StdValue (StandardT (StdIdT IO)))
+hnixEvalFile :: Options -> FilePath -> IO (StdValue (StandardT IO))
 hnixEvalFile opts file = do
   parseResult <- parseNixFileLoc file
   case parseResult of
@@ -36,11 +39,11 @@ hnixEvalFile opts file = do
             NixException frames ->
               errorWithoutStackTrace
                 .   show
-                =<< renderFrames @(StdValue (StandardT (StdIdT IO)))
-                      @(StdThunk (StandardT (StdIdT IO)))
+                =<< renderFrames @(StdValue (StandardT IO)) -- (StdValue (StandardT (FreshStableIdT IO)))
+                      @(StdThunk (StandardT IO) IO) -- (StdThunk (StandardT (FreshStableIdT IO)))
                       frames
 
-hnixEvalText :: Options -> Text -> IO (StdValue (StandardT (StdIdT IO)))
+hnixEvalText :: Options -> Text -> IO (NValue Identity (StandardT IO)) -- (StdValue (StandardT (FreshStableIdT IO)))
 hnixEvalText opts src = case parseNixText src of
   Failure err ->
     error
@@ -49,7 +52,7 @@ hnixEvalText opts src = case parseNixText src of
       <> "`.\n"
       <> show err
   Success expr ->
-    runWithBasicEffects opts $ normalForm =<< nixEvalExpr mempty expr
+    runWithBasicEffects opts $ normalForm =<< nixEvalExpr @Context @_ @(StandardT IO) Nothing expr
 
 nixEvalString :: String -> IO String
 nixEvalString expr = do

From 408093d7baa1deca141eb004e5a74c7d724c7931 Mon Sep 17 00:00:00 2001
From: Ali Abrar <aliabrar@gmail.com>
Date: Thu, 7 Jan 2021 20:17:59 -0500
Subject: [PATCH 2/4] Import transitional MonadFail where necessary for older
 GHCs

---
 src/Nix/Fresh/Stable.hs | 1 +
 src/Nix/Scope/Basic.hs  | 1 +
 src/Nix/Thunk/Basic.hs  | 1 +
 3 files changed, 3 insertions(+)

diff --git a/src/Nix/Fresh/Stable.hs b/src/Nix/Fresh/Stable.hs
index fe67ac8ae..7f9e9b72f 100644
--- a/src/Nix/Fresh/Stable.hs
+++ b/src/Nix/Fresh/Stable.hs
@@ -14,6 +14,7 @@ import Nix.Effects
 import Nix.Render
 import Nix.Thunk
 import Nix.Thunk.StableId
+import Control.Monad.Fail
 import Control.Monad.Reader
 import Control.Monad.State.Strict
 import Control.Monad.Ref
diff --git a/src/Nix/Scope/Basic.hs b/src/Nix/Scope/Basic.hs
index dd7f30431..900efe624 100644
--- a/src/Nix/Scope/Basic.hs
+++ b/src/Nix/Scope/Basic.hs
@@ -19,6 +19,7 @@ module Nix.Scope.Basic where
 
 import Control.Applicative
 import Control.Monad.Exception
+import Control.Monad.Fail
 import Control.Monad.Reader
 import Control.Monad.State
 import Control.Monad.Catch
diff --git a/src/Nix/Thunk/Basic.hs b/src/Nix/Thunk/Basic.hs
index 17aaefedb..a3c3191dc 100644
--- a/src/Nix/Thunk/Basic.hs
+++ b/src/Nix/Thunk/Basic.hs
@@ -18,6 +18,7 @@ module Nix.Thunk.Basic (ThunkT (..), runThunkT, NThunkF (..), Deferred (..)) whe
 
 import           Control.Exception       hiding ( catch )
 import           Control.Monad.Catch
+import           Control.Monad.Fail
 import           Control.Monad.Reader
 import           Control.Monad.State
 

From 98dc632fce4e2058812fcae0481eb2077b80ce14 Mon Sep 17 00:00:00 2001
From: Anton-Latukha <anton.latukha@gmail.com>
Date: Fri, 8 Jan 2021 21:26:54 +0200
Subject: [PATCH 3/4] load Control.Monad.Fail if GHC < 8.8

---
 src/Nix/Fresh/Stable.hs | 3 +++
 src/Nix/Scope/Basic.hs  | 3 +++
 src/Nix/Thunk/Basic.hs  | 3 +++
 3 files changed, 9 insertions(+)

diff --git a/src/Nix/Fresh/Stable.hs b/src/Nix/Fresh/Stable.hs
index 7f9e9b72f..908370cf5 100644
--- a/src/Nix/Fresh/Stable.hs
+++ b/src/Nix/Fresh/Stable.hs
@@ -14,7 +14,10 @@ import Nix.Effects
 import Nix.Render
 import Nix.Thunk
 import Nix.Thunk.StableId
+#if __GLASGOW_HASKELL__ < 880
+import Prelude hiding (fail)
 import Control.Monad.Fail
+#endif
 import Control.Monad.Reader
 import Control.Monad.State.Strict
 import Control.Monad.Ref
diff --git a/src/Nix/Scope/Basic.hs b/src/Nix/Scope/Basic.hs
index 900efe624..bf19ac6f9 100644
--- a/src/Nix/Scope/Basic.hs
+++ b/src/Nix/Scope/Basic.hs
@@ -19,7 +19,10 @@ module Nix.Scope.Basic where
 
 import Control.Applicative
 import Control.Monad.Exception
+#if __GLASGOW_HASKELL__ < 880
+import Prelude hiding (fail)
 import Control.Monad.Fail
+#endif
 import Control.Monad.Reader
 import Control.Monad.State
 import Control.Monad.Catch
diff --git a/src/Nix/Thunk/Basic.hs b/src/Nix/Thunk/Basic.hs
index a3c3191dc..119c400c9 100644
--- a/src/Nix/Thunk/Basic.hs
+++ b/src/Nix/Thunk/Basic.hs
@@ -18,7 +18,10 @@ module Nix.Thunk.Basic (ThunkT (..), runThunkT, NThunkF (..), Deferred (..)) whe
 
 import           Control.Exception       hiding ( catch )
 import           Control.Monad.Catch
+#if __GLASGOW_HASKELL__ < 880
+import           Prelude hiding (fail)
 import           Control.Monad.Fail
+#endif
 import           Control.Monad.Reader
 import           Control.Monad.State
 

From b60332575a5939da8f5bdcd4e75be42e8c3dde74 Mon Sep 17 00:00:00 2001
From: Anton-Latukha <anton.latukha@gmail.com>
Date: Tue, 9 Feb 2021 04:00:31 +0200
Subject: [PATCH 4/4] Eliminate use of QuantifiedConstraints to support GHC <
 8.6

---
 hnix.cabal             |  1 +
 src/Nix/Scope/Basic.hs |  1 +
 src/Nix/Standard.hs    | 11 ++++++++---
 src/Nix/Thunk.hs       | 15 +++++++++++++--
 src/Nix/Utils/Fix1.hs  |  7 +++++--
 5 files changed, 28 insertions(+), 7 deletions(-)

diff --git a/hnix.cabal b/hnix.cabal
index 847bb5444..cf8bd730e 100644
--- a/hnix.cabal
+++ b/hnix.cabal
@@ -404,6 +404,7 @@ library
     , binary >= 0.8.5 && < 0.9
     , bytestring >= 0.10.8 && < 0.11
     , comonad >= 5.0.4 && < 5.1
+    , constraints >= 0.11 && <0.13
     , containers >= 0.5.11.0 && < 0.7
     , data-fix >= 0.3.0 && < 0.4
     , deepseq >= 1.4.3 && <1.5
diff --git a/src/Nix/Scope/Basic.hs b/src/Nix/Scope/Basic.hs
index bf19ac6f9..983bbcd36 100644
--- a/src/Nix/Scope/Basic.hs
+++ b/src/Nix/Scope/Basic.hs
@@ -15,6 +15,7 @@
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE StandaloneDeriving #-}
+
 module Nix.Scope.Basic where
 
 import Control.Applicative
diff --git a/src/Nix/Standard.hs b/src/Nix/Standard.hs
index eaf7b1125..578ec92c7 100644
--- a/src/Nix/Standard.hs
+++ b/src/Nix/Standard.hs
@@ -8,7 +8,6 @@
 {-# LANGUAGE DeriveTraversable #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
-{-# LANGUAGE QuantifiedConstraints #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
@@ -30,6 +29,8 @@ import           Control.Monad.Reader
 import           Control.Monad.Ref
 import           Control.Monad.State
 import           Data.Coerce
+import           Data.Constraint                ( (\\) )
+import           Data.Constraint.Forall         ( Forall, inst )
 import           Data.Functor.Identity
 import           Data.HashMap.Lazy              ( HashMap )
 import           Data.Text                      ( Text )
@@ -173,8 +174,12 @@ instance HasCitations1 m v Identity where
 
 type StandardT m = Fix1T StandardTF m
 
-instance (forall m. MonadTrans (t (Fix1T t m))) => MonadTrans (Fix1T t) where
-  lift = Fix1T . lift
+class MonadTrans (t (Fix1T t m)) => TransAtFix1T t m
+
+instance MonadTrans (t (Fix1T t m)) => TransAtFix1T t m
+
+instance Forall (TransAtFix1T t) => MonadTrans (Fix1T t) where
+  lift (x :: m a) = Fix1T $ (lift \\ inst @(TransAtFix1T t) @m) x
 
 mkStandardT
   :: StandardTFInner (Fix1T StandardTF m) m a
diff --git a/src/Nix/Thunk.hs b/src/Nix/Thunk.hs
index cecb12cd4..79d9f8dc0 100644
--- a/src/Nix/Thunk.hs
+++ b/src/Nix/Thunk.hs
@@ -1,3 +1,7 @@
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE ApplicativeDo #-}
 {-# LANGUAGE LambdaCase #-}
@@ -18,6 +22,8 @@ import           Control.Monad.Trans.State
 import           Control.Monad.Trans.Writer
 import           Data.Typeable                  ( Typeable )
 import           Nix.Utils.Fix1
+import           Data.Constraint                ( (\\) )
+import           Data.Constraint.Forall         ( Forall, inst )
 
 class MonadTransWrap t where
   --TODO: Can we enforce that the resulting function is as linear as the provided one?
@@ -47,8 +53,13 @@ instance MonadTransWrap (StateT s) where
     put new
     pure result
 
-instance (forall m. MonadTransWrap (t (Fix1T t m))) => MonadTransWrap (Fix1T t) where
-  liftWrap f (Fix1T a) = Fix1T $ liftWrap f a
+
+class MonadTransWrap (t (Fix1T t m)) => TransWrapAtFix1T t m
+
+instance MonadTransWrap (t (Fix1T t m)) => TransWrapAtFix1T t m
+
+instance Forall (TransWrapAtFix1T t) => MonadTransWrap (Fix1T t) where
+  liftWrap (f :: forall x. m x -> m x) (Fix1T (a :: (t (Fix1T t m) m a))) = Fix1T $ liftWrap f a \\ inst @(TransWrapAtFix1T t) @m
 
 
 class ( Monad m
diff --git a/src/Nix/Utils/Fix1.hs b/src/Nix/Utils/Fix1.hs
index 4f853e66b..d8c72bfe1 100644
--- a/src/Nix/Utils/Fix1.hs
+++ b/src/Nix/Utils/Fix1.hs
@@ -7,7 +7,9 @@
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE StandaloneDeriving #-}
+{-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE UndecidableInstances #-}
 
 module Nix.Utils.Fix1 where
@@ -30,6 +32,7 @@ import           Control.Monad.Reader           ( MonadReader )
 import           Control.Monad.State            ( MonadState )
 
 
+
 -- | The fixpoint combinator, courtesy of Gregory Malecha.
 --   https://gist.github.com/gmalecha/ceb3778b9fdaa4374976e325ac8feced
 newtype Fix1 (t :: (k -> *) -> k -> *) (a :: k) = Fix1 { unFix1 :: t (Fix1 t) a }
@@ -50,6 +53,8 @@ deriving instance MonadState s (t (Fix1 t)) => MonadState s (Fix1 t)
 newtype Fix1T (t :: (k -> *) -> (* -> *) -> k -> *) (m :: * -> *) (a :: k)
   = Fix1T { unFix1T :: t (Fix1T t m) m a }
 
+type MonadFix1T t m = (MonadTrans (Fix1T t), Monad (t (Fix1T t m) m))
+
 deriving instance Functor (t (Fix1T t m) m) => Functor (Fix1T t m)
 deriving instance Applicative (t (Fix1T t m) m) => Applicative (Fix1T t m)
 deriving instance Alternative (t (Fix1T t m) m) => Alternative (Fix1T t m)
@@ -65,8 +70,6 @@ deriving instance MonadMask (t (Fix1T t m) m) => MonadMask (Fix1T t m)
 deriving instance MonadReader e (t (Fix1T t m) m) => MonadReader e (Fix1T t m)
 deriving instance MonadState s (t (Fix1T t m) m) => MonadState s (Fix1T t m)
 
-type MonadFix1T t m = (MonadTrans (Fix1T t), Monad (t (Fix1T t m) m))
-
 instance (MonadFix1T t m, MonadRef m) => MonadRef (Fix1T t m) where
   type Ref (Fix1T t m) = Ref m
   newRef  = lift . newRef