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?