[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:  closed
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.11
      Resolution:  invalid           |                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 simonpj):

 A difficulty is that in GHC today, each data constructor (such as `Cons`
 above) has a single, fixed memory layout.  So if we have
 {{{
 data T = MkT Bool {-# UNPACK !(Int, Int) #-}
 }}}
 we can decide to lay out `MkT` with a `Bool` field and two `Int` fields.

 But if we don't know `l`, we don't know how `HList l` is represented, so
 we can't do that.

 It's not just an implementation matter.  Suppose at an allocation site we
 were allocating  a `Cons` whose first arg was `'[Int,Bool]`.  Then you
 could imagine whizzing up a specialised `Cons` constructor.  But then
 `hlength` above would have to be able to consume that constructor!

 I don't know any way to do this without monomorphising the code.  Which is
 often, ''but not always'' possible, so you'd also need an escape hatch for
 the impossible case  --  and it's not obvious how to build the hatch.

 PhD thesis anyone?

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10048#comment:3>
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