[GHC] #1965: Allow unconstrained existential contexts in newtypes

[GHC]

#1965: Allow unconstrained existential contexts in newtypes
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        Reporter:  guest             |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  6.8.1
      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 Ericson2314):

 Yes for @sgraf's reason I wish we used an equivalence relation on these
 internal notion of deep representations, and also {{{RuntimeRep}}}s. For
 example, given matching deep structure (not tracked in the
 kind/{{{RuntimeRep}}}):
 {{{
 forall a t. exists t' t'. (t :: Kind a) ~R (t' :: Kind TupleRep [a]) ~R
 (t'' :: Kind SumRep [a])
 }}}

 Maybe it's not worth the extra coercions in practice, but certainly I
 think its good teaching to pretend newtypes are always:
 {{{
   newtype Foo (a :: k) :: TupleRep [k] = Foo (a :: k)
 }}}
 to explain
 {{{
 newtype A :: TupleRep [LiftedRep] = A (Int :: LiftedRep)
 case undefined of A _ -> 1
 }}}
 contrasted with pretend elaborated unpacked {{{!Int}}}:
 {{{
 data A' :: LiftedRep = A' (!Int :: IntRep)
 case undefined of A' _ -> undefined
 }}}
 That the shallow representations of {{{A}}} and {{{A'}}} have different
 strictness, opposite what is contained inside, cleanly explains how the
 `case`-expression has different semantics despite the overall isomorphism
 of {{{A}}} and {{{A'}}}.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/1965#comment:39>
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