[GHC] #14992: Allow existentials to UNPACK

[GHC]

#14992: Allow existentials to UNPACK
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        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      Resolution:  duplicate         |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by simonpj):

 * status:  new => closed
 * resolution:   => duplicate


Old description:

> Consider
> {{{
> data T a where
>   MkT :: b -> (b->a) -> T a
>
> data Wuggle where
>   MkW :: {-# UNPACK #-} !T Int -> Wuggle
> }}}
> Here `MkT` has an existential, but there is really no reason that it
> can't unpack into the data constructor for `MkW`.  But it is currently
> rejected.
>
> Fixing this would require quite a bit of new plumbing.  E.g. the
> existential type variable of the worker and wrapper would differ, which
> is not true today.  But morally it's the Right Thing.

New description:

 Consider
 {{{
 data T a where
   MkT :: b -> (b->a) -> T a

 data Wuggle where
   MkW :: {-# UNPACK #-} !T Int -> Wuggle
 }}}
 Here `MkT` has an existential, but there is really no reason that it can't
 unpack into the data constructor for `MkW`.  But it is currently rejected.

--

Comment:

 Ugh. Duplicate of #10016

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14992#comment:1>
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