#9017: Confusing error message with PolyKinds -------------------------------------+------------------------------------ Reporter: edsko | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by simonpj): I agree that this is bad. We really want more information about what goes wrong with the kind equality. At the moment, when we get a kind-incompatible type equality, such as `a:k ~ b:*`, we generate a "derived" kind equality `k ~ *`. In this case `k` is a skolem constant, so the constraint is insoluble. But derived constraints are discarded before error message generation (for good reason) so we only see the type-equality problem. We could instead generate a "wanted" kind equality; that would not be discarded, and we'd get a better error message: {{{ T9017.hs:8:7: Couldn't match kind `k1' with `*' `k1' is a rigid type variable bound by the type signature for foo :: a b (m b) at T9017.hs:7:8 When matching types a0 :: * -> * -> * a :: k1 -> k -> * Expected type: a b (m b) Actual type: a0 b0 (m0 b0) Relevant bindings include foo :: a b (m b) (bound at T9017.hs:8:1) In the expression: arr return In an equation for `foo': foo = arr return }}} Could still do with improvement (the foralls in the "bound by" message are invisible), but better. Sadly, a new wanted constraint should be used to solve the constraint that gave rise to it, otherwise we generate the same wanted constraint many times. And that is what happens here: we get two copies of the same error messages. With some faff we could elimate the dup, but I think it'd better to wait for Richard's upcoming work in which he gives us full-on kind equalities. Now we can use the kind equality to solve the type equality and all will be well. Meanwhile I attach my diff. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9017#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler