Factor out the abstract of which parsing library to use

Nix/Parser.hs

@@ -11,43 +11,11 @@ import qualified Data.Map as Map
 import           Data.Text hiding (concat, concatMap, head, map)
 import           Nix.Types
 import           Nix.Internal
+import           Nix.Parser.Library
 import qualified Prelude
 import           Prelude hiding (readFile, concat, concatMap, elem, mapM,
                                  sequence)
 
-#if USE_PARSEC
-import           Data.Text.IO
-import           Text.Parsec hiding ((<|>), many, optional)
-import           Text.Parsec.Text
-import           Text.PrettyPrint.ANSI.Leijen (Doc, text)
-
-symbol :: String -> Parser String
-symbol str = string str <* whiteSpace
-
-symbolic :: Char -> Parser Char
-symbolic c = char c <* whiteSpace
-
-decimal :: Parser Integer
-decimal = read <$> some digit
-
-whiteSpace :: Parser ()
-whiteSpace = spaces
-
-data Result a = Success a
-              | Failure Doc
-
-parseFromFileEx :: MonadIO m => Parser a -> FilePath -> m (Result a)
-parseFromFileEx p path =
-    (either (Failure . text . show) Success . parse p path)
-        `liftM` liftIO (readFile path)
-#else
-import           Text.Trifecta
-import           Text.Parser.LookAhead
-#endif
-
-parseNixFile :: MonadIO m => FilePath -> m (Result NExpr)
-parseNixFile = parseFromFileEx nixApp
-
 nixApp :: Parser NExpr
 nixApp = go <$> some (whiteSpace *> nixTerm True)
   where
@@ -174,3 +142,6 @@ setOrArgs = do
             trace ("args: " ++ show args) $ return ()
             symbolic ':' *> ((Fix .) . NAbs <$> pure args <*> nixApp)
                 <|> pure args
+
+parseNixFile :: MonadIO m => FilePath -> m (Result NExpr)
+parseNixFile = parseFromFileEx nixApp

Nix/Parser/Library.hs

@@ -0,0 +1,48 @@
+{-# LANGUAGE CPP #-}
+
+module Nix.Parser.Library
+    (
+#if USE_PARSEC
+      module Text.Parsec
+    , module Text.Parsec.Text
+#else
+      module Text.Trifecta
+    , module Text.Parser.LookAhead
+#endif
+    )
+    where
+
+#if USE_PARSEC
+
+import Control.Applicative
+import Data.Text.IO
+import Text.Parsec hiding ((<|>), many, optional)
+import Text.Parsec.Text
+import Text.PrettyPrint.ANSI.Leijen (Doc, text)
+
+symbol :: String -> Parser String
+symbol str = string str <* whiteSpace
+
+symbolic :: Char -> Parser Char
+symbolic c = char c <* whiteSpace
+
+decimal :: Parser Integer
+decimal = read <$> some digit
+
+whiteSpace :: Parser ()
+whiteSpace = spaces
+
+data Result a = Success a
+              | Failure Doc
+
+parseFromFileEx :: MonadIO m => Parser a -> FilePath -> m (Result a)
+parseFromFileEx p path =
+    (either (Failure . text . show) Success . parse p path)
+        `liftM` liftIO (readFile path)
+
+#else
+
+import Text.Parser.LookAhead
+import Text.Trifecta
+
+#endif

hnix.cabal

@@ -25,6 +25,7 @@ Library
     Nix.Types
   Other-modules:
     Nix.Internal
+    Nix.Parser.Library
   Default-extensions: 
     DataKinds
     DeriveDataTypeable