Re: [Haskell-cafe] Scope of type variables in associated types

Matthew Sackman wrote:

Andres Loeh wrote:

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class OneStep a data OS a :: * instance OneStep (Cons v t) data OS (Cons v t) = t

class TwoStep a data TS a :: * instance (OneStep a, OneStep b) => TwoStep a instance (OneStep a, OneStep (OS a)) => TwoStep a ?

Doesn't seem to work. Ok, my original was wrong as I had no constructor on the associated type. So below are 2 versions, one with fundeps, which works, one with associated type synonynms which doesn't work, which made me remember the warnings about how type synonynms aren't fully implemented yet, and so I wrote a third version with indexed types, but whilst I can make it work, wrapping up values in indexed types gets really really messy.

Yes, it's messy with indexed data families as they force you to introduce all these new constructors and the corresponding wrapping and unwrapping code. As Tom wrote in another message in this thread, you really do want to use indexed synonyms families (aka associated type synonyms) here. They will do what you want. We are currently working on completing the implementation of synonym families.[1] Manuel [1] For those who wonder why this is taking so long, we are working on a system that is actually significantly more general than what we described in the ICFP05 paper. In particular, we want synonym families to play nice with GADTs.