[Haskell-cafe] Type classes... popular for newbies, isn't it?
sholderm at students.cs.uu.nl
Sat Aug 7 02:29:58 EDT 2004
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
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