[GHC] #10343: Make Typeable track kind information better

[GHC]

#10343: Make Typeable track kind information better
-------------------------------------+-------------------------------------
        Reporter:  oerjan            |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  7.10.1
      Resolution:                    |             Keywords:  Typeable
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:  typeOf ::
                                     |  Typeable (a::k) => Proxy a ->
                                     |  TypeRep
      Blocked By:                    |             Blocking:
 Related Tickets:  #9858, #11011     |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by oerjan):

 From the discussion I've seen, I am purely guessing that all the necessary
 internal ''representations'' are now available, but the solver part is
 not. I.e. with `PolyKinds` enabled, the definition

 {{{
 {-# LANGUAGE PolyKinds #-}
 import Data.Typeable
 f :: Typeable a => Proxy a -> TypeRep
 f = typeOf
 }}}

 still won't compile, but you ''probably'' can now use the new `TypeRep`
 mechanisms do write an ''equivalent'' function by hand.

 On the other hand, since 8.0 with `TypeInType` you can write

 {{{
 f :: (Typeable (a::k), Typeable k) => Proxy a -> TypeRep
 f = typeOf
 }}}

 which means the actual ''need'' for this improvement is less than it was
 in 7.10.

 At the time I wrote this ticket, I was somewhat thinking about preserving
 backwards compatibility, which got broken in 7.10 anyhow.

 (And until recent trouble with the `constraints` package, I thought no one
 had been affected in practice - but the last version of that package
 actually now restricts the kind of the `Deferrable (a ~ b)` instance to
 `*` in just 7.10, because of this, see discussion at end of
 https://github.com/ekmett/constraints/issues/43.)

 Incidentally in GHCi 8.0.1, with `PolyKinds` but ''not'' `TypeInType`
 enabled:

 {{{
 Prelude Data.Typeable> let f x at Proxy = typeOf x
 Prelude Data.Typeable> :t f
 f :: forall k (t :: k).
      (Typeable * k, Typeable k t) =>
      Proxy k t -> TypeRep
 }}}

 which is convenient, but means you get an inferred type you cannot write.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10343#comment:38>
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