On Sep 29, 2012, at 9:49 PM, Gábor Lehel <illiss...@gmail.com> wrote:
> On Fri, Sep 28, 2012 at 6:36 PM, Francesco Mazzoli <f...@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:
>
> 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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>
>
>
> --
> Your ship was destroyed in a monadic eruption.
>
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