Fwd: UNPACK Existential datatype

[Alexander V Vershilov]

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?
>
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-- 
Alexander