Re: [Haskell-cafe] Type classes... popular for newbies, isn't it?

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