#16140: Cannot create type synonym for quantified constraint without ImpredicativeTypes -------------------------------------+------------------------------------- Reporter: Ashley Yakeley | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Resolution: | Keywords: | QuantifiedConstraints, | ImpredicativeTypes 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: | -------------------------------------+------------------------------------- Comment (by simonpj):
This proposal suggests accepting polytypes in all types of kind Constraint. Would that mean that the following would be accepted?
No, it would not. Under the `Proxy` we switch off the arbitrary-rank thing. Try it today! (The proposals doesn't change the behaviour of type signatures.
Would that count as impredicative?
I think we have decided that behaviour never changes when type synonyms are expanded; and have already gone to some trouble to make this so (in a recent ticket). So yes it would be impredicative, and would (or should) be caught. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16140#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler