Proposal: Improving the IsString String instance

[Reid Barton]

[Apologies to Henning and Edward for duplicate email]

On Sun, Aug 25, 2013 at 5:48 AM, Henning Thielemann <
schlepptop at henning-thielemann.de> wrote:

> Am 24.08.2013 19:52, schrieb Edward Kmett:
>
>
>  I would like to replace this instance with
>>
>> instance a ~ Char => IsString [a] where
>>    fromString = id
>>
>
>
> Your complaint proves my concerns about those FlexibleInstances. The best
> instance is a Haskell 98 instance:
>
>
> class IsCharList a where
>    fromCharList :: [a] -> String
>
> instance IsCharList Char where
>    fromCharList = id
>
> instance IsCharList a => IsString [a] where
>    fromString = fromCharList
>
>
> This is both the most flexible solution and it is portable.
>
> http://www.haskell.org/**haskellwiki/List_instance<http://www.haskell.org/haskellwiki/List_instance>


This doesn't compile (in the IsString instance, we need fromString ::
String -> [a], but we have fromCharList :: [a] -> String), and if you fix
the declaration of fromCharList to fromCharList :: String -> [a], it fails
to achieve the goal that was the original purpose of Edward's proposal.
 See below, where I've used RebindableSyntax to write a custom IsString
class to avoid overlapping with the built-in instance IsString String.

{-# LANGUAGE RebindableSyntax, OverloadedStrings #-}

import Prelude(Char, String, id, length, print)

class IsString a where
   fromString :: String -> a

class IsCharList a where
   fromCharList :: String -> [a]

instance IsCharList Char where
   fromCharList = id

instance IsCharList a => IsString [a] where
   fromString = fromCharList

main = print (length "abc")

{-
/tmp/is.hs:17:22:
    Ambiguous type variable `a0' in the constraint:
      (IsCharList a0) arising from the literal `"abc"'
    Probable fix: add a type signature that fixes these type variable(s)
    In the first argument of `length', namely `"abc"'
    In the first argument of `print', namely `(length "abc")'
    In the expression: print (length "abc")
-}

Regards,
Reid Barton
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