[Haskell-cafe] Class constraints with "free" type variables and fundeps

[MigMit]

Well, it seems that you can't do exactly what you want. So, the simplest way to do this would be not to make Foo a superclass for Bar:

class Bar a where
    foo :: Foo a b => a -> b -> c

Then you would have to mention Foo everywhere.

If you really need, for some reason, to ensure that every Bar instance has a corresponding Foo instance, you can do some oleging this way:

data Void b = Void
data FooEv a where FooEv :: Foo a b => Void b -> FooEv a
class Bar a where
    barFoo :: FooEv a
    bar :: Foo a b => a -> b -> c

Then, whenever you need Foo methods, you can do pattern-matching:

case barFoo :: FooEv a of
  FooEv (Void :: Void b) -> …

Now some "b" is in scope, and there is an instance of Foo a b.

On Sep 28, 2012, at 8:36 PM, Francesco Mazzoli <f at mazzo.li> wrote:

> I would expect this to work, maybe with some additional notation (a la
> ScopedTypeVariables)
> 
>    {-# LANGUAGE FunctionalDependencies #-}
>    {-# LANGUAGE MultiParamTypeClasses #-}
> 
>    class Foo a b | a -> b
> 
>    class Foo a b => Bar a where
>        foo :: a -> b -> c
> 
> The type family equivalent works as expected:
> 
>    {-# LANGUAGE TypeFamilies #-}
> 
>    class Foo a where
>        type T a :: *
> 
>    class Bar a where
>        foo :: a -> T a -> c
> 
> I can't use type families because the `Foo' I'm using is in an external library.
> Is there any way to achieve what I want without adding `b' to `Bar'?
> 
> --
> Francesco * Often in error, never in doubt
> 
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