[GHC] #15707: More liberally kinded coercions for newtypes

[GHC]

#15707: More liberally kinded coercions for newtypes
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        Reporter:  mniip             |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.6.1
  checker)                           |
      Resolution:                    |             Keywords:  Roles
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12369            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by mniip):

 Replying to [comment:2 goldfire]:
 > I see where you're getting, but it's still wishy-washy to me. Is there a
 concrete program you'd like to write with this feature?

 Me (and partly ekmett) are experimenting with the idea of building a
 polykinded generics framework using a type-level combinatory calculus. So
 there's a handful of such data families involved, acting as combinators.

 In practice of course the family can't be infinite and we have to stop at
 some arity. We could unsafely introduce a coercion of the desired shape,
 but then what if someone extends the family?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15707#comment:4>
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