[GHC] #8503: New GeneralizedNewtypeDeriving check still isn't permissive enough

[GHC]

#8503: New GeneralizedNewtypeDeriving check still isn't permissive enough
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        Reporter:  goldfire          |            Owner:  goldfire
            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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Changes (by simonpj):

 * cc: dimitris@…, sweirich@… (added)


Comment:

 Wait. Suppose we have `g1 : m ~R n`, and `g2 : a ~R b`. Then you might
 think we can conclude that `g1 g2 : m a ~R n b`.  But as you point out,
 our current rules say not.  (I'm looking in `docs/core-spec/core-
 spec.pdf`.)

 But ''why'' do they say not?  What does it mean for `m` to be
 representationally-equal to `n` (something that really only makes
 intuitive sense at kind *), other than "if you apply them to
 representationally-equal arguments you get representationally-equal types?

 Another alternative would be NOT to eta-reduce the newtype coercion.  See
 `Note [Newtype eta]` in `TyCon.lhs`.  It's much less important than it was
 because we are no longer coercing dictionaries.  But there would still be
 a cost, as the note points out.

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8503#comment:2>
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