[GHC] #11735: Optimize coercionKind

[GHC]

#11735: Optimize coercionKind
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 * In your `coercionRole`, `get_app` never uses its second argument!  So
 you can get rid of it entirely I think
 {{{
     go (InstCo co _) = go co
 }}}

 * `Note [Nested InstCos]` applies to `coercionKind` but not, I think, to
 `coercionRole`

 * `Note [Computing a coercion kind and role]` claims that computing the
 result of `coercionRole (NthCo d co)` requires `coercionKind`.  But it
 manifestly does not.  '''Richard''' is this note simply wrong?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11735#comment:19>
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