At 19:24 12/07/04 -0500, Ben.Yu@combined.com wrote:
Hi, please bear with me if my question turns out to be a stupid mistake. It has taken me hours to figure this out.
class Rule r u u' m where apply :: r -> u -> m u'
data And = And
data Bin a b o = Bin a b o
instance (Monad m, Rule r1 u u' m, Rule r2 u' u'' m) => Rule (Bin r1 r2 And) u u'' m where apply (Bin r1 r2 _) u = apply r1 u >>= apply r2
Ghc complains about "Could not deduce (Rule r1 u u'1 m, Rule r2 u'1 u'' m)", but it is obviously same as the constraint I gave in the instance declaration.
What am I doing wrong here?
There's no information in the instance type for the compiler to figure out what u' (in the constraint expressions) might be. (See other responses.) ... I would question whether or not you really should be using a type class here -- that depends on your application, but I've done some work involving simple inference rules and found that a polymorphic algebraic datatype (with function-valued members) served my purposes better than a type class. #g --
Thank you very much!
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