#15579: topNormaliseType is wrong -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: high | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I’m very puzzled about topNormaliseTYpe_maybe. Here it is: {{{ topNormaliseType_maybe env ty = topNormaliseTypeX stepper mkTransCo ty where stepper = unwrapNewTypeStepper `composeSteppers` tyFamStepper tyFamStepper rec_nts tc tys -- Try to step a type/data family = let (args_co, ntys) = normaliseTcArgs env Representational tc tys in -- NB: It's OK to use normaliseTcArgs here instead of -- normalise_tc_args (which takes the LiftingContext described -- in Note [Normalising types]) because the reduceTyFamApp below -- works only at top level. We'll never recur in this function -- after reducing the kind of a bound tyvar. case reduceTyFamApp_maybe env Representational tc ntys of Just (co, rhs) -> NS_Step rec_nts rhs (args_co `mkTransCo` co) _ -> NS_Done }}} I have two puzzlements 1. First, it seems utterly wrong to normalise the arguments using Representational. Consider {{{ F (N Int) where newtype N x = [x] }}} We don’t want to reduce `(N Int)` to `[Int]`, and then try reducing `(F [Int])`!! That is totally bogus. Surely we should use (the role-aware) `normalise_tc_args` here? 2. I have literally no clue what `Note [Normalising types]` is all about. Moreover there is no Lifting Context passed to `normalise_tc_args`, so I don’t know what this stuff about `LiftingContext` is about. Is this historical baggage? Richard and I discussed this. (1) is a bug; for (2) the comment is misleading and should be deleted. Richard will do these things -- and will add examples to the mysterious `Note [Normalising types]` -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15579 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler