Although it is impossible to create all necessary Typeable instances for
#9858: Typeable instances should be kind-aware -------------------------------------+------------------------------------- Reporter: dreixel | Owner: dreixel Type: bug | Status: new Priority: highest | Milestone: 7.10.1 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: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by int-e): I'm wondering about the ramifications of this claim from [[Typeable #Medium-termsolution]]: poly-kinded constructors at the definition site (there's an infinite number), it is possible to create Typeable "instances" on demand at use sites. Consider the following function: {{{ f = \(p :: Proxy Proxy) -> typeRep p }}} Because the kind information of the `p` argument is not fully known at the site of definition, I surmise that the corresponding `Kindable` (to use oerjan's nomenclature) constraint gets propagated outward, meaning that at the core language level, `f` gets an extra argument for passing the `Kindable` evidence for the unknown kind argument of the second `Proxy` type constructor. It is my understanding that for the sake of backward compatibility, this argument is implicit and not visible (even as a constraint) in the Haskell-level type. Is that correct? I feel uncomfortable about this; I like to be able to see in the Haskell type of functions what kind of runtime evidence needs to be passed to a function to make it work. The second thing I'm wondering about is what happens when polymorphic kinds are never instantiated at all. Is the following program valid, and if so, what will the kind of the `Proxy` be at runtime? {{{ {-# LANGUAGE PolyKinds, DataKinds #-} import Data.Typeable main :: IO () main = print (typeRep (Proxy :: Proxy Proxy)) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9858#comment:49 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler