[GHC] #13264: GHC panic with (->) generalization branch while compiling lens

[GHC]

#13264: GHC panic with (->) generalization branch while compiling lens
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        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
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):

 I have a terrible idea.

 Consider a unification variable (`MetaTv`) `alpha`. After `alpha` is
 filled in, say with `Int`, GHC should consider `alpha` and `Int` to be the
 same in all contexts. To achieve this, GHC must be very careful to zonk in
 just the right places. What if, instead, we use `unsafePerformIO` to zonk
 on the fly, whenever `coreView` is used? `coreView` looks through type
 synonyms, so it is currently put wherever we need to know about the "real"
 type -- in equality checks, splitting, etc. These places seem precisely
 the places where zonking is important. So we teach `coreView` how to zonk!
 Or, perhaps, we resurrect `tcView` (which was removed because it was just
 the same as `coreView`) so that this zonk-on-the-fly behavior happens only
 in the typechecker.

 Now, this idea (because it requires `unsafePerformIO`) goes against my
 grain. But what's actually wrong with it?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13264#comment:8>
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