#12749: Typechecker regression involving RankNTypes -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: high | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: | 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: | -------------------------------------+------------------------------------- Comment (by simonpj): It worked before due to a bug that makes type inference unpredictable. The issues it that the two branches of the `if` both have a higher rank type, and that is difficult. The solution here is to tell GHC the type of the conditional: {{{ applyNTCheck :: NatTrans (Either a) Maybe applyNTCheck = ((if b then trans else applyNT) :: NatTrans (Either a) Maybe -> NatTrans (Either a) Maybe) eitherToMaybe }}} Sorry! If I knew how to do better I would. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12749#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler