[GHC] #11636: Promoting newtype destructor

[GHC]

#11636: Promoting newtype destructor
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           Reporter:  phadej         |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 given
 {{{
 newtype LOL a = MkLOL { unLOL :: [[a]] }
 }}}

 I'd like to write
 {{{
 type family UnLOL (lol :: LOL a) = (result :: [[a]]) | result -> lol
 type instance UnLOL ('MkLOL xss) = xss
 }}}

 This is currently not accepted, but AFAICS it should be injective, as
 `lol` is kind restricted. Especially the example in injective type
 families paper do not apply, as `UnLOL Int` is ill-kinded.

 With this GHC could infer `lol ~ MkLOL xss` from `UnLOL lol ~ xss`, so
 newtypes would be useful for type-level programming, as type (kind)
 synonyms cannot be partially applied.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11636>
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