However, we would, of course, still like to find an instance where
#9948: Recommend class constraint instead of instance constraint -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.4 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by crockeea): Replying to [comment:5 goldfire]: possible. I see your point. So it's good that GHC looks at instances and tries to find a RHS match. If he constraints are also satisfied, then no errors should be reported. The case I'm talking about is when there is a matching instance [on the RHS], but the instance constraints are not satisfied. {{{ data T a class Foo b where foo :: b -> Int instance (Integral a) => Foo (T a) f :: T a -> Int f = foo }}} I don't like that GHC behaves differently in the presence of a matching instance vs when there are no matching instances. One workaround for what I'm suggesting (that I find myself frequently using) is to comment out all instances of the class. Then GHC just says `no instance for `Foo a` arising from the use of `foo`` instead of recommending constraints from the matching instance. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9948#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler