Data/String.hs

    {-# 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.
    --
    -----------------------------------------------------------------------------

    module Data.String (
       String
     , IsString(..)

     -- * Functions on strings
     , lines
     , words
     , unlines
     , unwords
     ) where

    import GHC.Base
    import Data.Functor.Const (Const (Const))
    import Data.Functor.Identity (Identity (Identity))
    import Data.List (lines, words, unlines, unwords)

    -- | Class for string-like datastructures; used by the overloaded string
    --   extension (-XOverloadedStrings in GHC).
    class IsString a where
        fromString :: String -> a

    {- Note [IsString String]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    Previously, the IsString instance that covered String was a flexible
    instance 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] for
    some t, but a flexible instance for [Char] will _only_ match if
    something further determines t to be Char, and nothing in the above
    example 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 simply
    typing:

       "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, overlaps
    with things that the [Char] flexible instance doesn't, but this was
    judged to be an acceptable cost, for the gain of providing a less
    confusing 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. If
    a 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, it
    is not clear at the time of this note exactly what such a system
    would be, and it certainly hasn't been implemented.

    A test case (should_run/overloadedstringsrun01.hs) has been added to
    ensure the good behavior of the above example remains in the future.
    -}

    -- | @(a ~ Char)@ context was introduced in @4.9.0.0@
    --
    -- @since 2.01
    instance (a ~ Char) => IsString [a] where
             -- See Note [IsString String]
        fromString xs = xs

    -- | @since 4.9.0.0
    deriving instance IsString a => IsString (Const a b)

    -- | @since 4.9.0.0
    deriving instance IsString a => IsString (Identity a)