[Haskell-cafe] default instance for IsString

[Markus Läll]

Hi Greg

You *can* have what you want -- enable extended defaulting and set
Text as default by

> {-# LANGUAGE ExtendedDefaultRules #-}
> default (Integer, Double, T.Text) -- keep the default defaults

At the moment you only by chance have one instance of NoDefault in
scope, which is why it shouldn't work at the moment.

(ExtendedDefaultRules is required because the standard defaulting
rules apply only when defaulting numeric literals.)

On Sun, Apr 22, 2012 at 7:55 AM, Greg Weber <greg at gregweber.info> 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
>>>>
>>>> _______________________________________________
>>>> Glasgow-haskell-users mailing list
>>>> Glasgow-haskell-users at haskell.org
>>>> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
>>>
>>>
>
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-- 
Markus Läll