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{-# LANGUAGE FlexibleInstances #-}{-# LANGUAGE GeneralizedNewtypeDeriving #-}{-# LANGUAGE NoImplicitPrelude #-}{-# LANGUAGE StandaloneDeriving #-}{-# LANGUAGE Trustworthy #-}{-# LANGUAGE TypeFamilies #-}------------------------------------------------------------------------------- |-- Module      :  Data.String-- Copyright   :  (c) The University of Glasgow 2007-- License     :  BSD-style (see the file libraries/base/LICENSE)---- Maintainer  :  libraries@haskell.org-- Stability   :  experimental-- Portability :  portable---- The @String@ type and associated operations.-------------------------------------------------------------------------------moduleData.String(String,IsString(..)-- * Functions on strings,lines,words,unlines,unwords)whereimportGHC.BaseimportData.Functor.Const(Const(Const))importData.Functor.Identity(Identity(Identity))importData.List(lines,words,unlines,unwords)-- | Class for string-like datastructures; used by the overloaded string--   extension (-XOverloadedStrings in GHC).classIsStringawherefromString::String->a{- Note [IsString String]~~~~~~~~~~~~~~~~~~~~~~~~~Previously, the IsString instance that covered String was a flexibleinstance for [Char]. This is in some sense the most accurate choice,but there are cases where it can lead to an ambiguity, for instance:  show $ "foo" ++ "bar"The use of (++) ensures that "foo" and "bar" must have type [t] forsome t, but a flexible instance for [Char] will _only_ match ifsomething further determines t to be Char, and nothing in the aboveexample actually does.So, the above example generates an error about the ambiguity of t,and what's worse, the above behavior can be generated by simplytyping:   "foo" ++ "bar"into GHCi with the OverloadedStrings extension enabled.The new instance fixes this by defining an instance that matches all[a], and forces a to be Char. This instance, of course, overlapswith things that the [Char] flexible instance doesn't, but this wasjudged to be an acceptable cost, for the gain of providing a lessconfusing experience for people experimenting with overloaded strings.It may be possible to fix this via (extended) defaulting. Currently,the rules are not able to default t to Char in the above example. Ifa more flexible system that enabled this defaulting were put in place,then it would probably make sense to revert to the flexible [Char]instance, since extended defaulting is enabled in GHCi. However, itis not clear at the time of this note exactly what such a systemwould be, and it certainly hasn't been implemented.A test case (should_run/overloadedstringsrun01.hs) has been added toensure the good behavior of the above example remains in the future.-}-- | @(a ~ Char)@ context was introduced in @4.9.0.0@---- @since 2.01instance(a~Char)=>IsString[a]where-- See Note [IsString String]fromString :: String -> [a]fromStringStringxs=[a]Stringxs-- | @since 4.9.0.0derivinginstanceIsStringa=>IsString(Constab)-- | @since 4.9.0.0derivinginstanceIsStringa=>IsString(Identitya)