ticket:3990#comment:56197 mentions the possibility of unpacking polymorphic fields. To quote:

[…]

What I mean by "polymorphic unpack" is this:

  data Poly a = MkP Bool {-# UNPACK #-} a 
  data Mango = MkMango {-# UNPACK #-} (Poly Int)

Now a value of type Poly t would be represented using two pointer fields, as usual (ie the UNPACK would have no direct effect on Poly). But a Mango value would be represented thus:

  data Mango = MkMangoRep Bool Int#

  MkMango :: Poly Int -> MangoRep
  MkMango (MkP b (I# i)) = MkMangoRep b i

  -- Pattern match      (MkMango p -> rhs)
  -- is transformed to  (MkMangoRep b i -> let p = MkP b (I# i) in rhs

Something like that would be rather nice. This ticket is just a reminder.

Thanks,[[br]] /Liyang