[GHC] #11011: Add type-indexed type representations (`TypeRep a`)

[GHC]

#11011: Add type-indexed type representations (`TypeRep a`)
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        Reporter:  bjmprice          |                Owner:
            Type:  feature request   |               Status:  patch
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2010
       Wiki Page:                    |
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Comment (by Iceland_jack):

 Bikeshedding.

 Base already has the existentials `SomeSymbol` and `SomeNat` so one
 wonders if `SomeTypeRep` is a more consistent name for `TypeRepX`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11011#comment:43>
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