#9730: Polymorphism and type classes -------------------------------------+------------------------------------- Reporter: mjaskel | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 (Type checker) | Keywords: Typeclass, Resolution: | Polymorphism, ImpredicativeTypes Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: GHC | Blocked By: rejects valid program | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by simonpj): I'm sorry but you really need impredicative polymorphism here, and GHC simply does not support it properly at the moment. (The `ImpredicativeTypes` stuff is badly broken.) However in this case there's an easy workaround {{{ a2c x = b2c (a2b x) }}} Now you don't need impredicativity Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9730#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler