Try class Rule r u u' m | r u -> u' Martin
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?
Thank you very much!
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