Fwd: UNPACK Existential datatype

Nicholas Clarke osymandias+glasgow-haskell-users at gmail.com
Tue Jan 20 13:08:14 UTC 2015

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

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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