#9195: ImpredicativeTypes and ConstraintKinds don't interact as expected ----------------------------+---------------------------------------------- Reporter: | Owner: MikeIzbicki | Status: new Type: bug | Milestone: Priority: normal | Version: 7.8.2 Component: Compiler | Operating System: Unknown/Multiple Keywords: | Type of failure: GHC rejects valid program Architecture: | Test Case: Unknown/Multiple | Blocking: Difficulty: Unknown | Blocked By: | Related Tickets: | ----------------------------+---------------------------------------------- Given this code: {{{ {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, ImpredicativeTypes, ConstraintKinds #-} class P a b instance P a b type Test a = forall b. P a b }}} I would expect to be able to actually use the Test Constraint in a function, like so: {{{ foo :: Test a => a -> a foo = id }}} But GHC complains that: {{{ Illegal polymorphic or qualified type: Test a In the type signature for ‘foo’: foo :: Test a => a -> a }}} I get the exact same error message if I replace ImpredicativeTypes with RankNTypes. If this is intended behavior under ImpredicativeTypes, how difficult would it be to add another extension that allows this type of polymorphism? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9195 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler