#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 goldfire): Replying to [comment:4 crockeea]:
I'm suggesting that if `foo` is a method of class `Foo`, then GHC simply says `cannot deduce (Foo a)`. There's no need to match instances at all.
However, we would, of course, still like to find an instance where possible. Just determining if an instance exists requires looking up instances for `Foo`, and then perhaps recurring on any constraints to those instances. At the end of a chain of instance constraints, we might end up with some constraint, say `Bar Int a`, that is unsatisfiable. Only then do we know that no instance matches the original constraint. To report a missing `Foo a` constraint at this point, we would need to somehow the need for `Bar Int a` came from a desire for `Foo a`. Keeping track of this is the "more work" I was talking about. Nowhere near impossible, but strictly harder than what happens currently. In any case, I think the discussion about how hard this feature is to implement detracts from the debate we should be having: what is really the desired behavior, regardless of implementation challenge. (To be clear, the implementation is not particularly challenging here, I think -- just not completely trivial.) And, regardless of implementation issues, I prefer the current behavior over the new one proposed in this ticket. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9948#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler