[GHC] #9858: Typeable instances should be kind-aware

[GHC]

#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:
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Comment (by oerjan):

 I was going to say that this need to do kind decomposition when inferring
 `Typeable` instances is why this cannot be done just with standalone
 instances, and you need a solver.

 In fact the last time I gave up on my [comment:33 attempt] to express this
 in source code, it was because I realized that the fundamental instance

 {{{
 instance (Typeable a, Typeable b) => Typeable (a b)
 }}}

 already needs such decomposition to be written.

 But today I had an epiphany: This ''can'' be expressed with current types,
 although with the annoying need to wrap kinds in proxies:

 {{{
 -- Ideally k itself should be the class argument, but that is not yet
 -- supported.  I think this is what makes UndecidableInstances
 -- necessary for defining instances.  Fortunately the instances seem to be
 -- usable without it.
 class (KindableParts a, a ~ Proxy) => Kindable (a :: k -> *) where
     type KindableParts a :: Constraint
     kindRep :: KindRep a

 class Kindable (Proxy :: k -> *) => Typeable (a :: k) where
     typeRep :: proxy a -> TypeRep
 }}}

 I wrote up enough of a mock implementation to make the last test case
 above work with this.
 The code is a bit long, so I put it
 [http://oerjan.nvg.org/haskell/TypeableDesign/ on my website].

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9858#comment:48>
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