lib: cleanup up megaparsec 6 compat module, simplify error type (#594)

hledger-lib/Text/Megaparsec/Compat.hs

@@ -1,3 +1,6 @@
+-- ^ Paper over some differences between megaparsec 5 and 6,
+-- making it possible to write code that supports both.
+
 {-# LANGUAGE CPP, FlexibleContexts #-}
 
 module Text.Megaparsec.Compat
@@ -24,7 +27,20 @@ import Data.List.NonEmpty (fromList)
 import Data.Void (Void)
 
 -- | A basic parse error type.
-type MPErr = ErrorFancy Void
+type MPErr = Void
+
+-- | Make a simple parse error.
+mpMkParseError :: FilePath -> String -> ParseError Char String
+mpMkParseError f s = FancyError (fromList [initialPos f]) (S.singleton $ ErrorFail s)
+
+-- | Make a Pos. With a negative argument, throws InvalidPosException (megaparsec >= 6)
+-- or calls error (megaparsec < 6).
+mpMkPos :: Int -> Pos
+mpMkPos = mkPos 
+
+-- | Unmake a Pos.
+mpUnPos :: Pos -> Int
+mpUnPos = unPos
 
 -- | Parse and return some Text.  
 mptext :: MonadParsec e Text m => Tokens Text -> m (Tokens Text) 
@@ -34,33 +50,24 @@ mptext = string
 
 import Text.Megaparsec.Prim (MonadParsec)
 
+-- | A basic parse error type.
 type MPErr = Dec
 
+-- | Make a simple parse error.
+mpMkParseError :: FilePath -> String -> ParseError Char String
+mpMkParseError f s = (mempty :: ParseError Char String){errorCustom = S.singleton $ f ++ ": " ++ s}
+
+-- | Make a Pos. With a negative argument, throws InvalidPosException (megaparsec >= 6)
+-- or calls error (megaparsec < 6).
+mpMkPos :: Int -> Pos
+mpMkPos = unsafePos . fromIntegral 
+
+-- | Unmake a Pos.
+mpUnPos :: Pos -> Int
+mpUnPos = fromIntegral . unPos 
+
+-- | Parse and return some Text.  
 mptext :: MonadParsec e Text m => Text -> m Text
 mptext = fmap pack . string . unpack
 
 #endif
-
-mpMkPos :: Int -> Pos
-mpMkPos = 
-#if MIN_VERSION_megaparsec(6,0,0)
-            mkPos
-#else
-            unsafePos . fromIntegral 
-#endif
-
-mpUnPos :: Pos -> Int
-mpUnPos = 
-#if MIN_VERSION_megaparsec(6,0,0)
-            unPos
-#else
-            fromIntegral . unPos 
-#endif
-
-mpMkParseError :: FilePath -> String -> ParseError Char String
-mpMkParseError f s = 
-#if MIN_VERSION_megaparsec(6,0,0)
-  FancyError (fromList [initialPos f]) (S.singleton $ ErrorFail s)
-#else
-  (mempty :: ParseError Char String){errorCustom = S.singleton $ f ++ ": " ++ s}
-#endif