#11674: GHC accepts overly general instance sigs -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: invalid | Keywords: InstanceSigs Operating System: Unknown/Multiple | Architecture: Type of failure: GHC accepts | Unknown/Multiple invalid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * status: new => closed * resolution: => invalid Comment: That's fine. It's equivalent to this {{{ instance Functor I where fmap = fmapI fmapI :: (a -> b) -> f a -> f b fmapI = undefined }}} So we check (a) that the defined function has the specified type, and (b) that a function of the specified type is acceptable in the instance decl. See 9.7.3.7 under http://downloads.haskell.org/~ghc/master/users- guide/glasgow_exts.html#instance-declarations. If the documentation should be clearer, could you suggest alternative wording that would have made it clearer? Thanks! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11674#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler