Arjan, AG> I'm curious as to why my class declaration AG> compiles in GHC, as there doesn't seem to AG> be any way to use it.

class (Ord st) => MinimaxState st where successors :: forall a . st -> [(a, st)] terminal :: st -> True

Any implementation of the successors method needs to produce values of an arbitrarely type a. Hence, it can only produce the empty list or a list of pairs that all have bottom as their first component.

instance MinimaxState Bool where successors = const [] terminal = not

instance MinimaxState Int where successors n = [(undefined, pred n), (undefined, succ n)] terminal 0 = True terminal n = False

HTH, Stefan

...

How about inserting one more parameter, "action", in your class definition: class (Ord st) => MinimaxState st action where successors:: st -> [(action,st)] terminal:: st -> Bool instance MinimaxState Int Int where terminal i = i == 0 successors i = [(1,i+1), (-1,i-1)]

I'd rather not do that, but even if I did, the type-variable action would not be reachable in the terminal function. I could specify a functional dependency st -> action (though I've never used it, it would be a fun to learn).

Right... you need the functional dependency because of "terminal", and in general a type annotation for using it. Carlos