[Haskell-cafe] Class constraints on associated types ...
aditya.siram at gmail.com
Fri Jan 2 18:59:52 UTC 2015
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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