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|
{-# LANGUAGE Trustworthy #-}
{-# LANGUAGE NoImplicitPrelude #-}
-----------------------------------------------------------------------------
-- |
-- 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 2010 'Read'. In particular, writing parsers is easier, and
-- the parsers are much more efficient.
--
-----------------------------------------------------------------------------
module Text.Read (
-- * 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 GHC.Base
import GHC.Read
import Data.Either
import Data.Maybe
import Text.ParserCombinators.ReadP as P
import Text.ParserCombinators.ReadPrec
import qualified Text.Read.Lex as L
------------------------------------------------------------------------
-- utility functions
-- | equivalent to 'readsPrec' with a precedence of 0.
reads :: Read a => ReadS a
reads = readsPrec minPrec
-- | 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
-- | 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)
|
