#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 simonpj): Reid is, as ever, spot on. I think it might not be hard for the error message to say {{{ foo.hs:15:11: Couldn't match type ‘a0’ with ‘a’ because type variable ‘a’ would escape its scope This (rigid, skolem) type variable is bound by the type signature for g :: a -> a at foo.hs:14:12-17 Expected type: a -> a Actual type: a0 -> a0 Relevant bindings include g :: a -> a (bound at foo.hs:15:7) f :: a0 -> a0 (bound at foo.hs:10:7) NB: let-bound f is monomorphic because of MonoLocalBinds <------------ In the expression: f In an equation for ‘g’: g = f }}} That is, annotate the "relevant bindings" that we did not attempt to generalise. It would be much harder to annotate only bindings that ''would'' generalise, but didn't. For example {{{ foo v = do let f x = v g :: a -> a g = f }}} Here `f` can't be generalised regardless of `MonoLocalBinds` because it mentions `v`. The other difficulty is that adding `-XNoMonoLocalBinds` if you have `-XGADTs` is extremely dodgy. (It should probably elicit a stern warning, although it does not at the moment.) So implicitly encouraging people to switch it off rather than fix the program properly is a bad idea. Perhaps the message should say: {{{ NB: Let-bound f was not generalised Possible solution: give it a type signature }}} Users, what do you think. Check out the [http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type- extensions.html#typing-binds user manual section] (7.13.9.3). Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9456#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler