[#1] internal functions documented

docs/index.md

@@ -3,7 +3,7 @@
 * [Compile-time options](compile-time-options.md): a set of options that may be used to change `cloak` behavior at compile-time
 * [Field types](field-types.md): description of different field types `cloak` supports
 * [Runtime errors](runtime-errors.md): runtime errors `cloak` can generate
-* [Module interface](module-interface.md): description of all exported functions cloak will generate for you
+* [Interface functions](interface-functions.md): description of all exported functions cloak will generate for you
 * [Types and specs](types-and-specs.md): exported generated types and function specifications
 * [Importers](importers.md): a way to import some fancy-packed data into your datastructure
 * [Validators](validators.md): different ways to validate data with user-defined callbacks

docs/interface-functions.md

@@ -0,0 +1,43 @@
+# Interface functions
+
+Interface functions are those functions that `cloak` will generate and export from your module.
+
+This section is mostly the [Getting Started](/README.md) guide from the main README file, but contains more details and links to the related documentation.
+
+## `new/1`
+
+`Module:new/1` is the most common way to create an opaque struct. It takes a map with `atom()` or `binary()` keys and `term()` values and returns an opaque record structure, which is obviously a `tuple()`:
+
+Note, that `new/1` totally ignores keys and values that are not presented in record definition, as well as your [private and protected](field-types.md) fields.
+
+If you will declare [datastructure-level importer](importers.md) for your datastructure, `new/1` will accept an argument of any type, not only maps. However, unsure you will return a map from your datastructure-level importer so that `cloak` will be able to perform ordinary initialization steps.
+
+This function also triggers all of [validators](validators.md) declared, both field-level and datastructure-level, in this order.
+
+## `update/2`
+
+`Module:update/2` function may be used to update several fields at a time. It basically acts in the same way as `new/1`, but threats all fields as optional. 
+
+While it will also ignore all unknown keys, as well as your [private and protected](field-types.md) fields, it performs per-field merging.
+
+This function will also trigger all [importers](importers.md) and all [validators](validators.md) declared.
+
+## `Getter/1`
+
+`cloak` will generate `Module:Getter/1` function for every [required, optional and protected](field-types.md) field declared.
+
+It is pretty straight-forward and simple returns `?#MODULE{}` record field value. 
+
+## `Setter/2`
+
+`cloak` will generate `Module:Getter/1` function for every [required and optional](field-types.md) field declared.
+
+```erlang
+7> cloak_example1:two(Opaque, 'CODE').
+{cloak_example1,hello,'CODE',generation}
+```
+
+## `export/1`
+
+`Module:export/1` returns your data as a map.
+