#7862: Could not deduce (A) from the context (A, ...) ----------------------------------------+----------------------------------- Reporter: alang9 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type checker) | Version: 7.6.2 Keywords: | Os: Linux Architecture: x86_64 (amd64) | Failure: GHC rejects valid program Difficulty: Unknown | Testcase: Blockedby: | Blocking: Related: | ----------------------------------------+----------------------------------- Comment(by simonpj): OK, so now we get (from the original program) {{{ T7862a.hs:17:24: Overlapping instances for Num (Tower s0 a) arising from a use of ‛<+>’ Matching givens (or their superclasses): (Num (Tower s a)) bound by the instance declaration at T7862a.hs:14:10-36 Matching instances: instance Num a => Num (Tower s a) -- Defined at T7862a.hs:19:10 (The choice depends on the instantiation of ‛a, s0’) In the expression: (Tower as) <+> (Tower as) }}} which is better but not good. For two reasons: * The "matching given" comes from one of the "silent supperclass" parameters, which is confusing to the user. * The whole thing is really due to the ambiguity of `s0`, and that's not even reported. The reason we are careful about overlap between givens and instances is desribed in `Note [Instance and Given overlap]` in `TcInteract`. I'm not sure what to do here. I'm expecting it's not a show-stopper for you: just add a type signature to resolve the ambiguity. -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7862#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler