#8388: forall on non-* types ------------------------------------+------------------------------------- Reporter: monoidal | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- This code {{{ {-# LANGUAGE RankNTypes, ImpredicativeTypes #-} x = Nothing :: (forall a. Maybe) Int }}} gives {{{ Couldn't match type `Maybe' with `forall a. Maybe' Expected type: forall a. Maybe Int Actual type: Maybe Int }}} which does not seem correct. There are two options: Option A: We allow this. Option B: We reject this as ill-kinded and allow `forall a. T` only when `T` has kind `*`. I prefer this option; in theory this can break existing code, but I am not aware of any code that uses forall on non-* type (please correct me if I'm wrong), and e.g. this means `forall a. a` inhabits every kind, for example `Bool`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8388 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler