#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 rwbarton): This is all expected behavior—for some value of "expected"! See https://www.haskell.org/ghc/docs/latest/html/users_guide/other-type- extensions.html#mono-local-binds. Your original program does not typecheck because - the `GADTs` language flag implies `MonoLocalBinds` - `f` does not have a user-supplied type signature - `f` cannot be floated to top-level because it refers to `box`, and... - `box` cannot be floated to top-level either because it is not bound by a `let` and therefore the type of `f` is not generalized, and so it cannot be used polymorphically by `g`. Your four other programs each differ in one of these four features. Granted, this is all not very obvious; do you have any suggestion for a better error message? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9456#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler