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

On Sep 29, 2012, at 9:49 PM, Gábor Lehel wrote:

On Fri, Sep 28, 2012 at 6:36 PM, Francesco Mazzoli 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:

http://hackage.haskell.org/trac/ghc/ticket/7100

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

type family FooFD a

Actually, I think it's better to use a class here: class Foo a (FooFD a) => FooProxy a where type 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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