#9456: Weird behavior with polymorphic function involving existential quantification and GADTs -------------------------------------+------------------------------------- Reporter: haasn | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 (Type checker) | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: GHC | Related Tickets: rejects valid program | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by haasn): My greatest concern with manually applying type signatures is that, as seen in my real world use case, adding an explicit type signature would require changing more than just that function, due to ScopedTypeVariables being needed. But, barring some way to tell GHC to explicitly “generalize (just) this function”, and assuming that adding -XNoMonoLocalBinds when it would otherwise be implied is a Bad Thing, then I'd be content with the user just being told to generalize manually. I do think adding a big fat warning label about something not generalizing is important, because I personally did not even know this restriction existed - and I would not have guessed adding a type signature fixes anything because I was operating under the impression that GHC already infers the most general rank-1 type for things that are not on the top level. I believe a false positive for things that could not generalize either way is an acceptable trade-off, as long as the message is not *too* obnoxious. In this particular case, I believe a good error message could also mention the fact that the type variable ‘a0’ is forced to be monomorphic, which is ultimately why ‘a’ can't be matched with ‘a0’ (which seems counter- intuitive given just the first line of the error message), but perhaps mentioning that the binding for ‘f :: a0 -> a0’ is monomorphic would suffice. Up to you. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9456#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler