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

[aditya siram]

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 <hesselink at gmail.com> wrote:

> Could you do something like this?
>
>     instance (Impl D1 D2 ~ a -> IO (), C a) => Dispatch D1 D2
>
> Erik
>
> On Fri, Jan 2, 2015 at 7:59 PM, aditya siram <aditya.siram at gmail.com>
> wrote:
> > 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
> 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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> > Haskell-Cafe at haskell.org
> > http://www.haskell.org/mailman/listinfo/haskell-cafe
> >
>
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