Language.Nix.Identifier: classify exposed symbols

- In general, one should use Identifier and its instances to render and
  parse Identifiers. quote has been relegated to an Internals section
  since it is only required by the test suite at the moment. It may be
  nice to remove it at some point.
- needsQuoting deals with arbitrary strings and would normally be used
  to judge whether a given String should be converted to an Identifier
  directly or first be subject to some kind of substitution so that it
  does not need to be quoted (see #672). nixKeywords is an internal
  definition involved with this which is also used in the test suites.

Author: sternenseemann

language-nix/src/Language/Nix/Identifier.hs

@@ -4,9 +4,15 @@
 {-# LANGUAGE TemplateHaskell #-}
 
 module Language.Nix.Identifier
-  ( Identifier, ident, quote, needsQuoting, nixKeywords
+  ( -- * Type-safe Identifiers
+    Identifier, ident
   , parseSimpleIdentifier, parseQuotedIdentifier
-  -- TODO: why do we expose quote?
+    -- * String Predicates
+  , needsQuoting
+    -- * Internals
+    -- TODO: only required by the language-nix test suite, unexport?
+  , nixKeywords
+  , quote
   )
   where
 
@@ -130,6 +136,7 @@ nixKeywords =
   [ "assert", "with", "if", "then", "else", "let", "in", "rec", "inherit", "or" ]
 
 -- | Helper function to quote a given identifier string if necessary.
+--   Usually, one should use the 'Pretty' instance of 'Identifier' instead.
 --
 -- >>> putStrLn (quote "abc")
 -- abc