Language.Nix.Identifier: quote Identifiers that are Nix keywords

This relates to #164, as there are packages like `assert` which clash with Nix keywords. However, this does not actually provide us with a solution since quoting isn't possible in some contexts (e.g. function arguments).
NixOS · sternenseemann · Sep 15, 2025 · Sep 12, 2025 · Sep 12, 2025 · Sep 12, 2025
commit 641787251eed74f7a44d30a3a8b3f992a4e9e6e2
diff --git a/language-nix/CHANGELOG.md b/language-nix/CHANGELOG.md
@@ -1,5 +1,13 @@
 # Revision History for language-nix
 
-## unreleased
+## 2.3.0 (unreleased)
 
+* `Language.Nix.Identifier` now exports `nixKeywords` which lists
+  keywords in Nix that are parseable as simple identifiers, but have
+  special meaning, like `assert` and `with`. Consequently, they can't
+  be used as (simple) identifiers in Nix code.
+
+  `quote`, `needsQuoting` and `Pretty` will take this list into account
+  and quote such identifiers. However, `HasParser` will _not_ reject them
+  even if they are unquoted.
 * Add an hspec/QuickCheck based test suite.
diff --git a/language-nix/language-nix.cabal b/language-nix/language-nix.cabal
@@ -1,5 +1,5 @@
 name:          language-nix
-version:       2.2.0
+version:       2.3.0
 synopsis:      Data types and functions to represent the Nix language
 description:   Data types and useful functions to represent and manipulate the Nix
                language.

diff --git a/language-nix/src/Language/Nix/Identifier.hs b/language-nix/src/Language/Nix/Identifier.hs
@@ -4,8 +4,9 @@
 {-# LANGUAGE TemplateHaskell #-}
 
 module Language.Nix.Identifier
-  ( Identifier, ident, quote, needsQuoting
+  ( Identifier, ident, quote, needsQuoting, nixKeywords
   , parseSimpleIdentifier, parseQuotedIdentifier
+  -- TODO: why do we expose quote?
   )
   where
 
@@ -44,6 +45,7 @@ import Text.PrettyPrint.HughesPJClass as PP
 -- __Warning__: Identifiers /may not/ contain @\'\\0\'@, but this is not
 -- checked during construction!
 --
+-- See also <https://nix.dev/manual/nix/2.30/language/identifiers.html>.
 declareLenses [d| newtype Identifier = Identifier { ident :: String }
                     deriving (Show, Eq, Ord, IsString, Generic)
               |]
@@ -75,10 +77,17 @@ instance CoArbitrary Identifier
 instance Pretty Identifier where
   pPrint = view (ident . to quote . to text)
 
+-- | Note that this parser is more lenient than Nix w.r.t. simple identifiers,
+--   since it will accept 'nixKeywords'.
 instance HasParser Identifier where
   parser = parseQuotedIdentifier <|> parseSimpleIdentifier
 
 -- | Parsec parser for simple identifiers, i.e. those that don't need quoting.
+--   The parser is equivalent to the regular expression @^[a-zA-Z_][a-zA-Z0-9_'-]*$@
+--   which the Nix parser uses.
+--
+--   Note that this parser will accept keywords which would not be parsed as
+--   identifiers by Nix, see 'nixKeywords'.
 parseSimpleIdentifier :: CharParser st tok m Identifier
 parseSimpleIdentifier = do
   c <- satisfy (\x -> x == '_' || isAlpha x)
@@ -104,10 +113,19 @@ parseQuotedIdentifier = Identifier <$> qstring
       return [c1,c2]
 
 -- | Checks whether a given string needs quoting when interpreted as an
--- 'Identifier'. Simple identifiers that don't need quoting match the
--- regular expression @^[a-zA-Z_][a-zA-Z0-9_'-]*$@.
+-- 'Identifier'.
 needsQuoting :: String -> Bool
-needsQuoting = isLeft . runParser (parseSimpleIdentifier >> eof) () ""
+needsQuoting s =
+  s `elem` nixKeywords
+  || isLeft (runParser (parseSimpleIdentifier >> eof) () "" s)
+
+-- | List of strings that are parseable as simple identifiers (see
+--   'parseSimpleIdentifier') in isolation, but won't be accepted by Nix because
+--   [keywords](https://nix.dev/manual/nix/2.30/language/identifiers.html#keywords)
+--   take precedence.
+nixKeywords :: [String]
+nixKeywords =
+  [ "assert", "with", "if", "then", "else", "let", "in", "rec", "inherit", "or" ]
 
 -- | Helper function to quote a given identifier string if necessary.
 --

diff --git a/language-nix/test/hspec.hs b/language-nix/test/hspec.hs
@@ -1,6 +1,7 @@
 module Main (main) where
 
 import Control.Lens
+import Control.Monad (forM_)
 import Data.String (fromString)
 import Language.Nix.Identifier
 import Test.Hspec
@@ -25,6 +26,11 @@ main = hspec $ do
       it "can parse the result of quote" $
         stringIdentProperty $ \str -> parseM "Identifier" (quote str) == Just (ident # str)
 
+    describe "nixKeywords" $ do
+      it "are quoted" $ forM_ nixKeywords $ \str -> do
+        shouldSatisfy str needsQuoting
+        prettyShow (ident # str) `shouldBe` "\"" ++ str ++ "\""
+
 stringIdentProperty :: (String -> Bool) -> Property
 stringIdentProperty p = property $ \s ->
   '\0' `notElem` s ==> classify (needsQuoting s) "need quoting" $ p s