#9017: Confusing error message with PolyKinds ------------------------------------+------------------------------------- Reporter: edsko | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- Consider the following example code: {{{ import Control.Arrow foo :: a b (m b) foo = arr return }}} if we typecheck this we rightly get two errors about missing instances (Arrow a, Monad m). However, if we enable `PolyKinds` we get the following error message: {{{ T.hs:4:7: Kind incompatibility when matching types: a0 :: * -> * -> * a :: k1 -> k -> * Expected type: a b (m b) Actual type: a0 b0 (m0 b0) Relevant bindings include foo :: a b (m b) (bound at T.hs:4:1) In the expression: arr return In an equation for ‘foo’: foo = arr return }}} I can ''sort of'' see where this is coming from if I think really hard :), so perhaps it's not a bug per se, but it's definitely confusing; and it's a pity that a type error (forgotten type qualifier) is reported as a kind error. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9017 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler