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! This message is intended only for the addressee and may contain information that is confidential or privileged. Unauthorized use is strictly prohibited and may be unlawful. If you are not the intended recipient, or the person responsible for delivering to the intended recipient, you should not read, copy, disclose or otherwise use this message, except for the purpose of delivery to the addressee. If you have received this email in error, please delete and advise us immediately.