[GHC] #15589: Always promoting metavariables during type inference may be wrong

[GHC]

#15589: Always promoting metavariables during type inference may be wrong
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 But how do we instantiate the type of `x` at an occurrence? We'll generate
 `alpha`, `beta`, and `delta` to instantiate for `a`, `b`, and `d`,
 respectively. We figured out a while ago that `a`'s kind -- hence
 `alpha`'s -- is `Type`. But what will `beta`'s kind be? It can't quite be
 `kappa[i34]`, because `kappa[i34]` might turn out to mention `a`, not
 `alpha`. Instead, it has to be `kappa[i34][a |-> alpha]`, as some kind of
 suspended substitution. My thesis (and Adam's) does this by including a
 list of variables at every occurrence of a unification variable. Thus, it
 would be `beta_[alpha]`, where it's understood that `alpha` is the
 instantiation of variables that might appear in `beta`'s kind. Similarly,
 `delta` would really be `delta_[alpha,beta]`, because `delta` is preceded
 by two instantiated variables.

 Really, all comment:1 does is to come up with a concise (and very
 convenient) way to store lists of variables, by naming the lists and
 storing the names. But I don't think it eliminates all the attendant
 complication of instantiation.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15589#comment:3>
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