Re: default instance for IsString

22 Apr 2012

      This is a better demonstration of the issue. I am going to open a GHC
bug report, as I can't see how this behavior is desirable.

{-# LANGUAGE OverloadedStrings #-}
import Data.Text as T

class    NoDefault a      where noDefault :: a -> Text
instance NoDefault T.Text where noDefault = id

main = print (noDefault "Hello!")

default.hs:7:15:
    Ambiguous type variable `a0' in the constraints:
      (NoDefault a0) arising from a use of `noDefault'
                     at default.hs:7:15-23
      (Data.String.IsString a0) arising from the literal `"Hello!"'
                                at default.hs:7:25-32
    Probable fix: add a type signature that fixes these type variable(s)
    In the first argument of `print', namely `(noDefault "Hello!")'
    In the expression: print (noDefault "Hello!")
    In an equation for `main': main = print (noDefault "Hello!")

On Sat, Apr 21, 2012 at 7:51 PM, Greg Weber  wrote:
...
my actual use case looks more like this:
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE FlexibleInstances, TypeSynonymInstances #-}
import Data.Text as T
class ShowT a where
  showT :: a -> String
instance ShowT T.Text where
  showT = show
instance ShowT String where
  showT = show
main = print (showT "Hello!")
   Ambiguous type variable `a0' in the constraints:
     (ShowT a0) arising from a use of `showT' at default.hs:16:15-19
     (Data.String.IsString a0) arising from the literal `"Hello!"'
So I actually want to define a default instance for a typeclass I
define that uses isString instances.
On Sat, Apr 21, 2012 at 6:24 PM, Daniel Peebles  wrote:
...
I think it'll be hard to do that without putting Text in base, which I'm not
sure anyone wants to do.
Dan
On Sat, Apr 21, 2012 at 8:20 PM, Greg Weber  wrote:
...
I would like to default IsString to use the Text instance to avoid
ambiguous type errors.
I see defaulting capability is available for Num. Is there any way to
do this for IsString?
Thanks,
Greg Weber
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