[GHC] #10016: UNPACK support for existentials

[GHC]

#10016: UNPACK support for existentials
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        Reporter:  simonpj           |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.8.4
      Resolution:                    |             Keywords:  DataCons
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 As far as I can tell, the only time this feature would be ''essential''
 would be a situation where you want to unpack an existential and don't
 have access to its constructor. That makes the "killer app" a bit tricky
 to find. On the other hand, it would certainly support the sum-of-records
 pattern:

 {{{#!hs
 data Foo = This | That !Bar
 data Bar = Bar { ... }
 }}}

 This lets people use named fields in sum types without introducing partial
 functions. As it is, it won't be efficient if `Bar` has any existentials.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10016#comment:5>
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