#8953: Reification drops necessary kind annotations -------------------------------------+------------------------------------- Reporter: goldfire | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: Component: Template | Version: 7.9 Haskell | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: | Related Tickets: None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by goldfire): Still getting worse! Now, look at this: {{{ a :: Proxy (Proxy :: * -> *) a = undefined b :: Proxy (Proxy :: (* -> *) -> *) b = undefined $( do info_a <- reify 'a info_b <- reify 'b runIO $ mapM putStrLn $ map pprint [info_a, info_b] return [] ) }}} produces {{{ Main.a :: Data.Proxy.Proxy Data.Proxy.Proxy Main.b :: Data.Proxy.Proxy Data.Proxy.Proxy }}} Yet, the types of `a` and `b` are ''not'' the same, and assigning one to the other rightly results in a type error. Furthermore, we really need to give the kind of ''every'' type variable binder. Consider {{{ type StarProxy (a :: *) = Proxy a }}} Reifying tyvar binders omits any kinds that are `*`, yet the kind annotation here is very relevant (if `-XPolyKinds` is on). Oh, and the original problem (not reifying kind annotation on type variable patterns) can affect ''class'' instances, too, not just ''type'' instances. So, new plan: 1. Never reify a tyvar binder into a `PlainTV`. Always use `KindedTV`. 2. Include a kind annotation (`SigT`) wrapping every use of a poly-kinded tycon. This wrapping will wrap around any arguments the tycon is applied to. 3. If a class or family tyvar is polykinded, include a kind annotation on any pattern used to specialize this tyvar in instance declarations. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8953#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler