[GHC] #11598: Cache coercion kinds and roles

[GHC]

#11598: Cache coercion kinds and roles
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           Reporter:  goldfire       |             Owner:  goldfire
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Because we never inspect the structure of a coercion, it is easy to cache
 coercions' kinds and role at the top level, using something like this:

 {{{
 data Coercion = MkCoercion { coercionKind :: Pair Type
                            , coercionRole :: Role
                            , coercionRep  :: CoercionRep }
 data CoercionRep = -- the long, recursive datatype we have now
 }}}

 In the process, various role and type information currently stored in the
 recursive structure could be dropped.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11598>
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