Re: [Haskell-cafe] Class constraints on associated types ...

Hi, That seemed to compile! I had no idea this kind of construction was even possible! However it did spew out a bunch of warnings like: "No explicit associated type or default declaration for ‘Impl’ in instance ..." Thanks! -deech On Fri, Jan 2, 2015 at 1:38 PM, Erik Hesselink wrote:

Could you do something like this?

instance (Impl D1 D2 ~ a -> IO (), C a) => Dispatch D1 D2

Erik

...

Hi all, I'd like to be able to constrain an associated type.

I used to have an instance that looked like: class Dispatch a b c | a b -> c where runF :: a -> b -> c instance (C a) => Dispatch D1 D2 ( a -> IO ()) where runF d1 d2 = (\_ -> return ())

Since I upgraded to 7.8 from 7.5 that instance declaration is no longer accepted is no longer accepted since it violates FD's.

I have been updating my code to use type families like so: class Dispatch a b where type Impl a b :: * runF :: a -> b -> Impl a b instance (C a) => Dispatch D1 D2 where type Impl D1 D2 = a -> IO () runF d1 d2 = (\_ return ())

Unfortunately the `type Impl ...` line in the instance is rejected because it uses `a` on the RHS.

In this one case I could just package it up into a newtype or something but I would ideally like to be able to constrain any number of arguments

On Fri, Jan 2, 2015 at 7:59 PM, aditya siram wrote: like:

...

instance (C a, C b ... C z) => Dispatch D1 D2 where type Impl D1 D2 = a -> b -> ... -> z -> IO () ...

This was something I could do in 7.6 (although I realize this is way safer). How do I go about getting that constraint back?

Thanks! -deech

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