[GHC] #14728: Is (GeneralizedNewtypeDeriving + associated type classes) completely bogus?

[GHC]

#14728: Is (GeneralizedNewtypeDeriving + associated type classes) completely bogus?
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler (Type    |              Version:  8.2.2
  checker)                           |             Keywords:  deriving,
      Resolution:  fixed             |  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4402
       Wiki Page:                    |
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Comment (by RyanGlScott):

 I completely take back what I said in comment:15 about this being
 unblocked—that couldn't be further from the truth! In fact, after talking
 with goldfire and kcsongor about this, we've come to the realization that
 all of the ideas in comment:3 are completely untenable at present.

 The issue is that we're trying to construct the type `(x |> g)`, where `x`
 is a type and `g` is a coercion. However, in order for this to kind-check,
 `g` must be nominally roled. This is never the case in GND, as we always
 use newtype axioms, which are by definition representationally roled!

 This pretty much makes this idea dead in the water, at least until we
 figure out a way to have representational casts in kinds (which is likely
 a ways away).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14728#comment:16>
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