#11675: Monomoprhic code makes ImpredicativeTypes infer an existential type -------------------------------------+------------------------------------- Reporter: _deepfire | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1-rc2 checker) | Resolution: invalid | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by goldfire): * status: new => closed * resolution: => invalid Comment: <snarky-joke>Yes, there is an outright bug: your module includes `{-# LANGUAGE ImpredicativeTypes #-}`.</snarky-joke> GHC's behavior is according to published typing rules, such as those in my recent [http://www.seas.upenn.edu/~sweirich/papers/esop2016-typeapp.pdf paper] on the subject. The problem is that nothing encourages GHC to instantiate the expression `Nothing` in your first case branch, and so GHC doesn't. If you add a type annotation there (or a visible type application) the problem should go away. (But I can't test this at the moment -- sorry.) It is regrettable that `ImpredicativeTypes` is not conservative over Haskell98, but the extension is known to be quite broken and is hopefully getting an overhaul soon. See ImpredicativePolymorphism. Thanks for reporting! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11675#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler