[Haskell-cafe] Class constraints on associated types ...
hesselink at gmail.com
Fri Jan 2 19:38:13 UTC 2015
Could you do something like this?
instance (Impl D1 D2 ~ a -> IO (), C a) => Dispatch D1 D2
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?
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