[Haskell-cafe] What can be UNPACK'ed?

[Tom Ellis]

What types can be UNPACK'ed?  From my vague understanding of what UNPACK
does, it can only be types whose size is bounded by a known value, so whilst

    data Foo = Foo {-# UNPACK #-} !Float {-# UNPACK #-} !Int

is fine, presumably

    data Foo = Foo {-# UNPACK #-} ![Float]

is not fine.  Or is it?  Technically it could be possible to reserve enough
space in Foo to store either a [] or a (:) Float [Float].  And if that is
possible could we also UNPACK polymorphic fields?

    data Storable a => Foo a = Foo {-# UNPACK #-} a

or

    data Foo a where
        Foo :: Storable a => {-# UNPACK #-} a -> Foo a

if either DatatypeContexts worked or my imaginary GADT UNPACK syntax worked
(I'm not sure if either does).

What are the limits of what we could reasonably get to UNPACK in GHC?

Thanks,

Tom