{-# 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)