#9858: Typeable instances should be kind-aware -------------------------------------+------------------------------------- Reporter: dreixel | Owner: Type: bug | Status: merge Priority: highest | Milestone: 7.10.2 Component: Compiler | Version: 7.9 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | typecheck/should_fail/T9858a, | should_run/T9858b | Blocking: | Differential Revisions: Phab:D652 -------------------------------------+------------------------------------- Comment (by simonpj): Sigh. `ImpredicativeTypes` certainly causes a lot of pain given that it's a feature that doesn't work and is unsupported. The problem here is that we should never unify `(c => t)` with `a b`, as happens in the call `i (Proxy :: Proxy (Eq Int => Int))`. Here what happens when I fix this: {{{ T9858.hs:24:8: error: Couldn't match type `Eq Int => Int' with `a0 b0' Expected type: Proxy (a0 b0) Actual type: Proxy (Eq Int => Int) In the first argument of `i', namely `(Proxy :: Proxy (Eq Int => Int))' In the expression: i (Proxy :: Proxy (Eq Int => Int)) In an equation for `j': j = i (Proxy :: Proxy (Eq Int => Int)) }}} And that seems entirely reasonable. Thank you - you are amazingly good at exposing my inadequate thinking. Keep it up! (Meanwhile I'll plug this hole tomorrow.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9858#comment:103 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler