#10644: Option GADTs specialises a polymorphic value in local bindings -------------------------------------+------------------------------------- Reporter: bales | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: invalid | Keywords: Operating System: Linux | Architecture: x86_64 Type of failure: GHC rejects | (amd64) valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Changes (by simonpj): * status: new => closed * resolution: => invalid Comment: This is just a consequence of `MonoLocalBinds`, as specified in [https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/other- type-extensions.html#mono-local-binds 7.13.9.3 of the user manual]. GADTs switches on `MonoLocalBinds`, so the definition of `p` is not generalised, and that in turn leads to the error you encountered. You might reasonably think that GADTs have nothing do to with generalisation, but alas they do, as the papers mentioned in the manual describe. I'm not saying it's brilliant behaviour, just that I don't know how to do better. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10644#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler