[GHC] #15009: Float equalities past local equalities

[GHC]

#15009: Float equalities past local equalities
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  feature request   |               Status:  closed
        Priority:  normal            |            Milestone:  8.4.3
       Component:  Compiler          |              Version:  8.2.2
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:  gadt/T15009
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by nfrisby):

 I think I'm slowly making progress! I'm doing it at wiki:FloatMoreEqs2018,
 to avoid polluting this ticket any more. I might open a separate feature
 request at some point.

 In particular, I work through an example there

 {{{
 forall[3] y. () =>
   ( (u : <U>)
   , forall[4]. (g : y ~ <Y>)   =>   (w1 : <W1>)
   )
 }}}

 showing why floating {{{w1}}} out from under {{{g}}} can be problematic. I
 think this answers my question from comment:10 more precisely than does
 comment:11.

 That example motivates a rule, which I'm now working to broaden and
 concretize:

 > We can float {{{w1}}} out from under {{{g}}} if {{{y}}} does not and can
 never again occur in {{{<W1>}}}.

 The destination I have in mind is something like "retiring" {{{g}}} (e.g.
 discarding it) if we can determine that it has eliminated all possible
 occurrences of {{{y}}} under it now and forever. I think some relatively
 simple heuristics are within reach -- will keep working on it.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15009#comment:19>
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