Fwd: UNPACK Existential datatype

[Alexander V Vershilov]

Please take a took at that commit, UNPACK was also handled there, despite
commit message do not explicitly state this.
On Jan 23, 2015 11:49 AM, "Roman Cheplyaka" <roma at ro-che.info> wrote:

> How is parsing of the *NOUNPACK* pragma relevant here?
>
> On 23/01/15 10:45, Alexander V Vershilov wrote:
> > Hi.
> >
> > As far as I understand  it was fixed as:
> >
> > commit 1d32a8503c2ebfab2bbdb696fe65dd0823d1ed27
> > Author: Simon Peyton Jones <simonpj at microsoft.com>
> > Date:   Mon Dec 1 17:07:48 2014 +0000
> >
> >     Fix parser for UNPACK pragmas
> >
> >        {-# NOUNPACK #-}
> >        {-# NOUNPACK #-} !
> >     were being parsed the same way.  The former was wrong.
> >
> >     Thanks to Alan Zimmerman for pointing this out
> >
> >
> > So it will fix is in 7.10. And I can't reproduce this anymore on
> > ghc-HEAD.
> >
> >
> > On 20 January 2015 at 17:35, Roman Cheplyaka <roma at ro-che.info> wrote:
> >> Interesting question. I managed to trace this to:
> >>
> >> compiler/basicTypes/MkId.hs:699
> >>
> >> isUnpackableType fam_envs ty
> >>   | Just (tc, _) <- splitTyConApp_maybe ty
> >>   , Just con <- tyConSingleAlgDataCon_maybe tc
> >>   , isVanillaDataCon con
> >>   = ok_con_args (unitNameSet (getName tc)) con
> >>   | otherwise
> >>   = False
> >>
> >> where isVanillaDataCon is defined as:
> >>
> >> dcVanilla :: Bool,
> >> -- True <=> This is a vanilla Haskell 98 data constructor
> >> --          Its type is of form
> >> --              forall a1..an . t1 -> ... tm -> T a1..an
> >> --          No existentials, no coercions, nothing.
> >>
> >> There's no explanation why this limitation is introduced; it might be
> >> just a conservative one.
> >>
> >> On 20/01/15 15:08, Nicholas Clarke wrote:
> >>> I'd like to be able to use the UNPACK pragma on an existentially
> >>> quantified datatype. So as in the below example:
> >>>
> >>> {-# LANGUAGE ExistentialQuantification #-}
> >>>
> >>> data Foo = forall a. Show a => Foo !a
> >>> instance Show Foo where
> >>>   show (Foo a) = "Foo! " ++ show a
> >>>
> >>> data Bar =
> >>>     Bar {-# UNPACK #-} !Foo
> >>>   deriving (Show)
> >>>
> >>> main :: IO ()
> >>> main = do
> >>>   let foo = Foo "Hello"
> >>>       bar = Bar foo
> >>>   print bar
> >>>
> >>> I would expect the `Foo` constructor to be unpacked into Bar, as if I
> >>> had written:
> >>>
> >>> data Bar = forall a. Show a => Bar !a
> >>>
> >>> However, instead I get the 'Ignoring unusable UNPACK pragma on the
> first
> >>> argument of ‘Bar’' warning. Is there a reason this shouldn't work, or a
> >>> workaround to get it to do so?
> >>
> >> _______________________________________________
> >> Glasgow-haskell-users mailing list
> >> Glasgow-haskell-users at haskell.org
> >> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
> >
> >
> >
>
>
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