Re: [Haskell-cafe] Disambiguating a Num/RealFrac instance

28 May 2015

      On 28/05/15 18:28, Brandon Allbery wrote:
...
On Tue, May 26, 2015 at 8:35 PM, mailto:amindfv@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-...

Hope this helps,

Adam

-- 
Adam Gundry, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com/