[GHC] #15996: Add Unlifted List type to base

[GHC]

#15996: Add Unlifted List type to base
-------------------------------------+-------------------------------------
           Reporter:  chessai        |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.6.3
          Component:                 |           Version:  8.6.2
  libraries/base                     |
           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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 {{{#!hs
 data UList (a :: TYPE 'UnliftedRep) where
   UNil :: UList a
   UCons :: a -> UList a -> UList a
 }}}

 This would guarantee that values stored inside the list would not be
 thunks. It would likely live in something like GHC.List.Unlifted, since it
 uses GHC-specific things.

 An example of something it might improve is the implementation of
 Control.Concurrent.QSem.QSem:

 {{{#!hs
 data QSem = QSem !(MVar (Int, [MVar ()], [MVar ()]))
 }}}

 this could instead be:

 {{{#!hs
 type MVarIO = MVar# RealWorld -- type synonym for brevity

 data QSem = QSem (MVarIO (Int, UList (MVarIO ()), UList (MVarIO ())))
 }}}

 Note that in this example the tuple inside the outermost `MVarIO` boxes
 the `Int` - this tuple could be represented as a datatype which ensures
 the unboxing on the `Int`.

 The main idea here though is that now you can use `MVar#` inside of the
 `UList`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15996>
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