[Haskell-cafe] Optimizing unamb by determining the "state" of a thunk?

I was wandering if it would be possible to optimize unambhttp://hackage.haskell.org/packages/archive/unamb/0.1.9/doc/html/Data-Unamb.... by checking if a value is already evaluated to head normal form. So f `unamb` g would then be extremely fast if either f or g is already evaluated to head normal form. Maybe using some vacuum tricks? This function would need to be in IO since it is of course not referentially transparent. Although threads are lightweight in Haskell, forking/waiting/killing surely must have more overhead than just checking the thunk of an expression? Of course one could also make unamb a primitive :-)