Re: [Haskell] type class does not compile
At 19:24 12/07/04 -0500, [EMAIL PROTECTED] 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! 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. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell Graham Klyne For email: http://www.ninebynine.org/#Contact ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
[Haskell] type class does not compile
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. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
[Haskell] type class does not compile
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! 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. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
[Haskell] type class does not compile
Another solution would be to use lexically scoped type variables: 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)::m u') = apply r2 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! 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. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
