Extra.hs

{-# LANGUAGE CPP #-}
{-# OPTIONS_GHC -fno-warn-duplicate-exports #-}

-- | This module provides "Text.Read" with functions added in later versions.
module Text.Read.Extra(
    module Text.Read,
    readEither, readMaybe
    ) where

import Text.Read

#if __GLASGOW_HASKELL__ < 706

import 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.
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.
readMaybe :: Read a => String -> Maybe a
readMaybe s = case readEither s of
                Left _  -> Nothing
                Right a -> Just a

#endif