#9858: Typeable instances should be kind-aware -------------------------------------+------------------------------------- Reporter: dreixel | Owner: Type: bug | Status: new Priority: highest | Milestone: 7.12.1 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: Phab:D652 -------------------------------------+------------------------------------- Comment (by oerjan): Replying to [comment:45 goldfire]:
I believe a sufficiently smart solver should be able to do this without difficulty.
After all the discussion way above, I'll just point out now that the current implementation isn't one of those. I guess it was too much to hope for in such a hurried situation. For example, it breaks on the following expression (with `PolyKinds`, thanks to Shachaf): {{{ typeOf :: Typeable a => Proxy a -> TypeRep }}} It seems here that it cannot deduce `Typeable (Proxy a)` from `Typeable a` when the kind isn't known at compile time. More seriously, I think the current `TypeRep` for `Typeable (a :: k)` doesn't contain enough information to extract the `KindRep` for `k`, much less to decompose it, even if the solver were to try. It seems to have been designed such that type application of known `Typeable` types (with known kinds) will work almost effortlessly, but anything fancier is impossible. Ironically I thought (with my convoluted attempts to represent everything in 7.8 types) that type application would be the main thing forcing kind decomposition to be implemented for handling currently existing code, but this representation simply bypasses that. And I guess if no one else has complained then that is currently enough for people's needs. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9858#comment:79 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler