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?

Cheers,

Nick