[GHC] #8095: TypeFamilies painfully slow

[GHC]

#8095: TypeFamilies painfully slow
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        Reporter:  MikeIzbicki       |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.4.1
       Component:  Compiler (Type    |              Version:  7.6.3
  checker)                           |
      Resolution:                    |             Keywords:  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #5321, #11598,    |  Differential Rev(s):  Phab:D3752
  #12506                             |
       Wiki Page:                    |
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Comment (by simonpj):

 I'd be very reluctant to have a complicated solution that was hard to be
 sure was correct here.  (e.g. tracking covasrs in `FlattenEnv`.)

 I'm surprised that finding the free variables of a coercion is so
 expensive.

 Zonking traerses coercions anyway.  Suppose the result of zonking a
 coercion was always `UnivCo t1 t2 free-vars`.  Then to zonk `Trans c1 c2`
 you can zonk `c1` and `c2`, and unition their free vars.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8095#comment:43>
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