Well, `=>` is not really a type-constructor, but rather a syntactic
#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 oerjan): Replying to [comment:98 diatchki]: form. So I don't think it should ever appear alone, or partially applied, in a Haskell program. It doesn't have to appear literally alone. You can already extract a partially applied form, although seemingly not the "constructor" itself. {{{ {-# LANGUAGE ImpredicativeTypes #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE FlexibleContexts #-} import Data.Typeable f :: Proxy (a b) -> Proxy a f _ = Proxy g = f (Proxy :: Proxy (Eq Int => Int)) h :: Proxy a -> Proxy (a Bool) h _ = Proxy i = h g }}} {{{ *Main> :t g g :: Proxy (* -> *) ((->) (Eq Int)) *Main> :t i i :: Proxy * (Eq Int => Bool) }}} Once you have a partially applied form, you can get `Typeable` instances for the parts, and then compose them, as shown in [comment:90 comment 90]. Interestingly, GHC internally ensures (and I don't know whether this is a bug or an intended feature) that if you try to extract the `(=>)` *itself*, you get `(->) :: * -> * -> *` instead, and a kind matching error. The same applies to `->` if you try to pattern match away an unlifted argument type. But in both cases, you can pattern match away a final *lifted* argument type to get a partially applied `(->)` with an argument of strange kind. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9858#comment:99 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler