[Apologies to Henning and Edward for duplicate email]

On Sun, Aug 25, 2013 at 5:48 AM, Henning Thielemann <schlepptop@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

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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