Re: Fwd: UNPACK Existential datatype

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 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 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@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users