[GHC] #8707: Kind inference fails in data instance definition

[GHC]

#8707: Kind inference fails in data instance definition
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        Reporter:  goldfire          |            Owner:  jstolarek
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.7
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by goldfire):

 Quite right. But, your comment about the fact that this works for data
 instances gives me an idea: what about doing this unification in
 `kcDataDefn`? Note that the function is used only in the data instance
 case. (It might be worth renaming to make this now-critical fact more
 obvious.) There's no knot to worry about there. `kcConDecl` would be
 rewritten to return the desugared result type so that `kcDataDefn` can
 access it. Should be rather straightforward.

 If you agree that this might work, the question now is: is it better to
 have this (admittedly somewhat delicate) check in addition to the already-
 delicate handling of GADT return types? Or is it better to have a
 duplication of all the work in `tc_hs_type`? If we go with duplicating
 `tc_hs_type`, it's not immediately obvious exactly how the GADT-return-
 type typechecker would operate, given that the constraints on that type
 may appear arbitrarily deep in the type, depending on the data instance
 type patterns. I'm sure there's a way to get it to work, but I have a
 sneaking suspicion it will look a lot like a call to `unifyType`
 somewhere.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8707#comment:13>
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