#10789: Notify user when a kind mismatch holds up a type family reduction -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- Consider this contrived example: {{{ {-# LANGUAGE TypeFamilies, PolyKinds, UndecidableInstances #-} module Bug where import Data.Proxy type family F (a :: k) :: k type instance F a = G a type family G a type instance G a = a foo :: Proxy (F Maybe) -> Proxy Maybe foo = id }}} This (correctly) fails to compile. The error message is {{{ Bug.hs:14:7: Couldn't match type ‘F Maybe’ with ‘Maybe’ Expected type: Proxy (F Maybe) -> Proxy Maybe Actual type: Proxy Maybe -> Proxy Maybe In the expression: id In an equation for ‘foo’: foo = id Failed, modules loaded: none. }}} But this is peculiar, but it surely looks like `F` should be a type-level identity function! Of course, upon further inspection, we see that `F` is partial. It reduces only at kind `*`. This is quite hard to figure out, though, especially given that we're using the "default to `*`" behavior of open type families to arrange for this kind restriction. Thus, I propose: figure out when a type family reduction is held up due to a kind mismatch, and inform the user. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10789 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler