Re: [Haskell-cafe] Rank-2 types in classes

Thank you ! Is what I'm trying to do a common technique to type-ensure contexts or are there simpler methods? 2011/3/2 Max Bolingbroke

On 2 March 2011 09:11, Yves Parès wrote:

...

class (forall x. Monad (IM i x)) => Impl i where data IM i :: * -> * -> *

But GHC forbids me to do so.

The way I usually work around this is by doing something like the following pattern:

{{{ class Monad1 m where return1 :: a -> m x a bind1 :: m x a -> (a -> m x b) -> m x b

instance Monad1 (IM MyI) where return1 = ... bind1 = ...

instance Monad1 m => Monad (m x) where return = return1 (>>=) = bind1 }}}

Your class can now have a (Monad1 (IM i)) superclass context. You will have to enable a few extensions to get this through - most likely FlexibleInstances and OverlappingInstances.

Cheers, Max