Compat.hs

{-# LANGUAGE CPP, NoImplicitPrelude #-}
module Text.Read.Compat (
   -- * The 'Read' class
   Read(..),
   ReadS,

   -- * Haskell 2010 functions
   reads,
   read,
   readParen,
   lex,

   -- * New parsing functions
   module Text.ParserCombinators.ReadPrec,
   L.Lexeme(..),
   lexP,
   parens,
   readListDefault,
   readListPrecDefault,
   readEither,
   readMaybe

 ) where

import Text.Read
import Text.ParserCombinators.ReadPrec
import qualified Text.Read.Lex as L

#if !(MIN_VERSION_base(4,6,0))
import Prelude.Compat
import qualified Text.ParserCombinators.ReadP as P

-- | Parse a string using the 'Read' instance.
-- Succeeds if there is exactly one valid result.
-- A 'Left' value indicates a parse error.
--
-- /Since: 4.6.0.0/
readEither :: Read a => String -> Either String a
readEither s =
  case [ x | (x,"") <- readPrec_to_S read' minPrec s ] of
    [x] -> Right x
    []  -> Left "Prelude.read: no parse"
    _   -> Left "Prelude.read: ambiguous parse"
 where
  read' =
    do x <- readPrec
       lift P.skipSpaces
       return x

-- | Parse a string using the 'Read' instance.
-- Succeeds if there is exactly one valid result.
--
-- /Since: 4.6.0.0/
readMaybe :: Read a => String -> Maybe a
readMaybe s = case readEither s of
                Left _  -> Nothing
                Right a -> Just a
#endif