ch10.hs

module Ch10 where

import Data.Time

data DatabaseItem = DbString   String
                    | DbNumber Integer
                    | DbDate   UTCTime
                    deriving (Eq, Ord, Show)

theDatabase :: [DatabaseItem]
theDatabase =
  [ DbDate (UTCTime
            (fromGregorian 1911 5 1)
            (secondsToDiffTime 34123))
  , DbNumber 9001
  , DbNumber 9002
  , DbNumber 9002
  , DbString "Hello World"
  , DbDate (UTCTime
              (fromGregorian 1921 5 1)
              (secondsToDiffTime 34123))
  ]

filterDbDate :: [DatabaseItem] -> [UTCTime]
filterDbDate [] = []
filterDbDate (DbDate date:xs) = date : filterDbDate xs
filterDbDate (_:xs) = filterDbDate xs

filterDbNumber :: [DatabaseItem] -> [Integer]
filterDbNumber [] = []
filterDbNumber (DbNumber number:xs) = number : filterDbNumber xs
filterDbNumber (_:xs) = filterDbNumber xs

mostRecent :: [DatabaseItem] -> UTCTime
mostRecent db = foldr max (head dates) (tail dates)
  where dates = filterDbDate $ db

sumDb :: [DatabaseItem] -> Integer
sumDb db = foldr (+) 0 $ filterDbNumber db

avgDb :: [DatabaseItem] -> Double
avgDb db = fromIntegral (sumDb db) / fromIntegral (length $ filterDbNumber db)