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

Gábor Lehel illissius at gmail.com
Sat Sep 29 19:49:36 CEST 2012

On Fri, Sep 28, 2012 at 6:36 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'?

I was browsing the GHC bug tracker and accidentally might have found a
solution to your problem:


Basically you have to make a type family to recapitulate the
functional dependencies in the instances of Foo:

type family FooFD a

-- for each instance Foo A B, you have to write:
-- type instance FooFD A = B

class Foo a (FooFD a) => Bar a where
    foo :: a -> FooFD a -> c

Anywhere you would use 'b', you use the type family instead.

The example in the ticket also had a 'b ~ FooFD a' superclass
constraint on Foo itself, which you can't add if you don't control
Foo, but I'm not sure what it's necessary for - in my brief tests
removing it didn't cause problems.

A weakness of this approach is that you have to manually add a type
instance for every instance of Foo, which may or may not be a problem
in your situation.

> --
> Francesco * Often in error, never in doubt
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Your ship was destroyed in a monadic eruption.

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