Text/Read.hs

{-# OPTIONS_GHC -XNoImplicitPrelude #-}
-----------------------------------------------------------------------------
-- |
-- Module : Text.Read
-- Copyright : (c) The University of Glasgow 2001
-- License : BSD-style (see the file libraries/base/LICENSE)
-- 
-- Maintainer : libraries@haskell.org
-- Stability : provisional
-- Portability : non-portable (uses Text.ParserCombinators.ReadP)
--
-- Converting strings to values.
--
-- The "Text.Read" library is the canonical library to import for
-- 'Read'-class facilities. For GHC only, it offers an extended and much
-- improved 'Read' class, which constitutes a proposed alternative to the 
-- Haskell 98 'Read'. In particular, writing parsers is easier, and
-- the parsers are much more efficient.
--
-----------------------------------------------------------------------------

module Text.Read (
 -- * The 'Read' class
 Read(..), -- The Read class
 ReadS, -- String -> Maybe (a,String)

 -- * Haskell 98 functions
 reads, -- :: (Read a) => ReadS a
 read, -- :: (Read a) => String -> a
 readParen, -- :: Bool -> ReadS a -> ReadS a
 lex, -- :: ReadS String

#if defined(__GLASGOW_HASKELL__) || defined(__HUGS__)
 -- * New parsing functions
 module Text.ParserCombinators.ReadPrec,
 L.Lexeme(..),
 lexP, -- :: ReadPrec Lexeme
 parens, -- :: ReadPrec a -> ReadPrec a
#endif
#ifdef __GLASGOW_HASKELL__
 readListDefault, -- :: Read a => ReadS [a]
 readListPrecDefault, -- :: Read a => ReadPrec [a]
#endif

 ) where

#ifdef __GLASGOW_HASKELL__
import GHC.Base
import GHC.Read
import Data.Either
import Text.ParserCombinators.ReadP as P
#endif
#if defined(__GLASGOW_HASKELL__) || defined(__HUGS__)
import Text.ParserCombinators.ReadPrec
import qualified Text.Read.Lex as L
#endif

#ifdef __HUGS__
-- copied from GHC.Read

lexP :: ReadPrec L.Lexeme
lexP = lift L.lex

parens :: ReadPrec a -> ReadPrec a
parens p = optional
 where
 optional = p +++ mandatory
 mandatory = do
 L.Punc "(" <- lexP
 x <- reset optional
 L.Punc ")" <- lexP
 return x
#endif

#ifdef __GLASGOW_HASKELL__
------------------------------------------------------------------------
-- utility functions

-- | equivalent to 'readsPrec' with a precedence of 0.
reads :: Read a => ReadS a
reads = readsPrec minPrec

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

-- | The 'read' function reads input from a string, which must be
-- completely consumed by the input process.
read :: Read a => String -> a
read s = either error id (readEither s)
#endif