#12749: Typechecker regression involving RankNTypes -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: high | Milestone: Component: Compiler | Version: 8.0.1 (Type checker) | 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: -------------------------------------+------------------------------------- The following code compiles on GHC 7.0.4 through 7.10.3, but not with GHC 8.0.1, 8.0.2, or HEAD: {{{#!hs {-# LANGUAGE RankNTypes #-} module Bug where type NatTrans f g = forall x. f x -> g x -- NatTrans analog of id applyNT :: NatTrans f g -> NatTrans f g applyNT x = x b :: Bool b = True eitherToMaybe :: NatTrans (Either a) Maybe eitherToMaybe (Left _) = Nothing eitherToMaybe (Right x) = Just x trans :: NatTrans (Either a) Maybe -> NatTrans (Either a) Maybe trans x = x applyNTCheck :: NatTrans (Either a) Maybe applyNTCheck = (if b then trans else applyNT) eitherToMaybe }}} {{{ $ /opt/ghc/8.0.1/bin/ghc Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) Bug.hs:21:27: error: • Couldn't match type ‘NatTrans (Either a) Maybe’ with ‘Either a0 x0 -> Maybe x0’ Expected type: (Either a0 x0 -> Maybe x0) -> Either a x -> Maybe x Actual type: NatTrans (Either a) Maybe -> Either a x -> Maybe x • In the expression: trans In the expression: if b then trans else applyNT In the expression: (if b then trans else applyNT) eitherToMaybe • Relevant bindings include applyNTCheck :: Either a x -> Maybe x (bound at Bug.hs:21:1) }}} (Originally spotted [https://github.com/ku-fpg/natural- transformation/pull/13#issuecomment-255378141 here].) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12749 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler