[Haskell] type class does not compile

Martin Sulzmann sulzmann at comp.nus.edu.sg
Mon Jul 12 20:51:50 EDT 2004

Try class Rule r u u' m | r u -> u'


 > 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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