{-# 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.-------------------------------------------------------------------------------moduleText.Read(-- * The 'Read' classRead (..),ReadS ,-- * Haskell 2010 functionsreads ,read ,readParen ,lex ,-- * New parsing functionsmoduleText.ParserCombinators.ReadPrec ,L.Lexeme (..),lexP ,parens ,readListDefault ,readListPrecDefault ,readEither ,readMaybe )whereimportGHC.Base importGHC.Read importData.Either importText.ParserCombinators.ReadP asPimportText.ParserCombinators.ReadPrec importqualifiedText.Read.Lex asL-- $setup-- >>> import Prelude-------------------------------------------------------------------------- utility functions-- | equivalent to 'readsPrec' with a precedence of 0.reads ::Read a =>ReadS a reads :: forall a. Read a => ReadS a
reads =Int -> ReadS a
forall a. Read a => Int -> ReadS a
readsPrec Int
minPrec -- | Parse a string using the 'Read' instance.-- Succeeds if there is exactly one valid result.-- A 'Left' value indicates a parse error.---- >>> readEither "123" :: Either String Int-- Right 123---- >>> readEither "hello" :: Either String Int-- Left "Prelude.read: no parse"---- @since 4.6.0.0readEither ::Read a =>String ->Either String a readEither :: forall a. Read a => String -> Either String a
readEither String
s =case[a
x |(a
x ,String
"")<-ReadPrec a -> Int -> ReadS a
forall a. ReadPrec a -> Int -> ReadS a
readPrec_to_S ReadPrec a
read' Int
minPrec String
s ]of[a
x ]->a -> Either String a
forall a b. b -> Either a b
Right a
x []->String -> Either String a
forall a b. a -> Either a b
Left String
"Prelude.read: no parse"[a]
_->String -> Either String a
forall a b. a -> Either a b
Left String
"Prelude.read: ambiguous parse"whereread' :: ReadPrec a
read' =doa
x <-ReadPrec a
forall a. Read a => ReadPrec a
readPrec ReadP () -> ReadPrec ()
forall a. ReadP a -> ReadPrec a
lift ReadP ()
P.skipSpaces a -> ReadPrec a
forall a. a -> ReadPrec a
forall (m :: * -> *) a. Monad m => a -> m a
return a
x -- | Parse a string using the 'Read' instance.-- Succeeds if there is exactly one valid result.---- >>> readMaybe "123" :: Maybe Int-- Just 123---- >>> readMaybe "hello" :: Maybe Int-- Nothing---- @since 4.6.0.0readMaybe ::Read a =>String ->Maybe a readMaybe :: forall a. Read a => String -> Maybe a
readMaybe String
s =caseString -> Either String a
forall a. Read a => String -> Either String a
readEither String
s ofLeft String
_->Maybe a
forall a. Maybe a
Nothing Right a
a ->a -> Maybe a
forall a. a -> Maybe a
Just a
a -- | The 'read' function reads input from a string, which must be-- completely consumed by the input process. 'read' fails with an 'error' if the-- parse is unsuccessful, and it is therefore discouraged from being used in-- real applications. Use 'readMaybe' or 'readEither' for safe alternatives.---- >>> read "123" :: Int-- 123---- >>> read "hello" :: Int-- *** Exception: Prelude.read: no parseread ::Read a =>String ->a read :: forall a. Read a => String -> a
read String
s =(String -> a) -> (a -> a) -> Either String a -> a
forall a c b. (a -> c) -> (b -> c) -> Either a b -> c
either String -> a
forall a. String -> a
errorWithoutStackTrace a -> a
forall a. a -> a
id (String -> Either String a
forall a. Read a => String -> Either String a
readEither String
s )