[Haskell-cafe] Disambiguating a Num/RealFrac instance

[amindfv at gmail.com]

Oh apologies -- it looks like my contrived example was a little too contrived. I'm actually using a MPTC, so the type would be more like:

class ToGPA a b where
  toGPA :: a -> GPA

ExtendedDefaultRules doesn't work with MPTCs it seems.


Tom


El May 28, 2015, a las 13:43, Adam Gundry <adam at well-typed.com> escribió:

> [Sorry, CCing haskell-cafe rather than ghc-devs!]
> 
> On 28/05/15 18:28, Brandon Allbery wrote:
>> On Tue, May 26, 2015 at 8:35 PM, <amindfv at gmail.com
>> <mailto:amindfv at gmail.com>> wrote:
>> 
>>   Is there any way (without IncoherentInstances or Rebindablesyntax)
>>   that I can let the user write e.g. "giveGPA 4.0" (and "giveGPA 4")
>>   and get back "F 4" without getting type errors that "4.0"'s type is
>>   ambiguous? I can guarantee there won't be any additional instances
>>   to "ToGPA"
>> 
>> 
>> A typeclass with only one instance is nonsensical, and often a symptom
>> of trying to use typeclasses as OO classes. All it's doing here is
>> hurting you.
> 
> Like Brandon, I suspect this probably isn't what you should do. But if
> you *really* want to do it, this works:
> 
>   {-# LANGUAGE ExtendedDefaultRules, FlexibleInstances #-}
> 
>   default (Float)
> 
>   data GPA = F Float | Excuse String
> 
>   class ToGPA a where
>     giveGPA :: a -> GPA
> 
>   instance ToGPA Float where
>     giveGPA = F
> 
>   instance ToGPA String where
>     giveGPA = Excuse
> 
>   x = giveGPA 4
>   y = giveGPA 4.0
>   z = giveGPA "Hello"
> 
> For more information:
> https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/interactive-evaluation.html#extended-default-rules
> 
> Hope this helps,
> 
> Adam
> 
> 
> -- 
> Adam Gundry, Haskell Consultant
> Well-Typed LLP, http://www.well-typed.com/