> The meaning of a program is its behavior, period. If reality doesn't agree with your views, it's your fault, not reality's.

That's not the usual meaning of, well, meaning.

> Standard ML doesn't give you any way to get the memory address of a particular value, so values that are semantically identical are actually indistinguishable in practice.

Just tried it to be sure:

    fun address i = Unsafe.cast(ref i) : Int32.int ;;
    val a = 4 :: nil ;;
    val b = 4 :: nil ;;
    address (a) ;;
    address (b) ;;
    address (a) ;;

And yep, a and b are semantically identical values but distinguishable in practice:

    val address = fn : 'a -> Int32.int
    val a = [4] : int list
    val b = [4] : int list
    val it = ~175003712 : Int32.int
    val it = ~174998232 : Int32.int
    val it = ~175003712 : Int32.int