signature MLTON_WEAK =
   sig
      type 'a t

      val get: 'a t -> 'a option
      val new: 'a -> 'a t
   end

A weak pointer is a pointer to an object that is nulled if the object becomes unreachable due to garbage collection. The weak pointer does not itself cause the object it points to be retained by the garbage collector -- only other strong pointers can do that. For objects that are not allocated in the heap, like integers, a weak pointer will always be nulled. So, if w: int Weak.t then Weak.get w = NONE.

• type 'a t
  the type of weak pointers to objects of type 'a

• get w
  returns NONE if the object pointed to by w no longer exists. Otherwise, returns SOME of the object pointed to by w.

• new x
  returns a weak pointer to x.