#11319: ImpredicativeTypes even more broken than usual -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.11 checker) | Keywords: Resolution: | ImpredicativeTypes Operating System: Linux | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by rwbarton): * cc: goldfire (added) Comment: Nothing special about deriving here, try: {{{ {-# LANGUAGE ImpredicativeTypes #-} f :: Applicative f => f (Maybe a) f = pure Nothing main = return () {- Error.hs:4:5: error: • Couldn't match type ‘forall a1. Maybe a1’ with ‘Maybe a’ Expected type: f (Maybe a) Actual type: f (forall a. Maybe a) • In the expression: pure Nothing In an equation for ‘f’: f = pure Nothing • Relevant bindings include f :: f (Maybe a) (bound at Error.hs:4:1) -} }}} This is with an up-to-the-minute version of HEAD, that contains the relevant-looking Phab:1715. As `ImpredicativeTypes` is unsupported anyways, perhaps we should just take the opportunity to kill it? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11319#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler