{-# 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-------------------------------------------------------------------------- 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.---- >>> 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 s =case[x |(x ,"")<-readPrec_to_S read' minPrec s ]of[x ]->Right x []->Left "Prelude.read: no parse"_->Left "Prelude.read: ambiguous parse"whereread' =dox <-readPrec lift P.skipSpaces return 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 s =casereadEither s ofLeft _->Nothing Right a ->Just 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 s =either errorWithoutStackTrace id (readEither s )