default instance for IsString

[Brent Yorgey]

I do not think this is a bug.  Since type classes are open, GHC does
not do any reasoning of the form "X is the only instance in scope, so
I will pick that one".  Other instances could be added at any time
(perhaps in other modules).  In this particular instance, GHC has no
reason to choose the Text instance other than the fact that it is the
only instance in scope -- that is, type inference is not enough to
determine that the Text instance should be chosen.

However, I do agree that it would be nice to have a mechanism for
specifying default instances for arbitrary (user-defined) type
classes.

-Brent

On Sat, Apr 21, 2012 at 09:55:32PM -0700, Greg Weber wrote:
> 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 <greg at gregweber.info> 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 <pumpkingod at gmail.com> 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 <greg at gregweber.info> 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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> >>> Glasgow-haskell-users at haskell.org
> >>> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
> >>
> >>
> 
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