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 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, 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/