[GHC] #10048: {-# UNPACK #-} support for size-indexed types

[GHC]

#10048: {-# UNPACK #-} support for size-indexed types
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        Reporter:  dfeuer            |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.11
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by rwbarton):

 How would we implement

 {{{
 hlength :: HList l -> Int
 hlength Nil = 0
 hlength (Cons _ xs) = 1 + hlength xs
 }}}

 then? (Bear in mind that we have no access to the type parameter `l` at
 runtime.)

 I think this sort of thing could only work with a type family rather than
 a GADT (and even then I haven't thought about whether it actually could
 work).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10048#comment:1>
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