Does SWI-Prolog do variable aliasing?

Help!
1

When issuing the following code at the prompt…

?- X is 3, Y is 3.

The following output appears:

X = Y, Y = 3.

I’d love a better understanding of what’s happening with that X=Y, and why it doesn’t say X=3. Is SWI-Prolog doing some kind of variable aliasing as an optimization?

2

Variable aliasing? No, because the variables are immutable.

Unification? Yes, Prolog uses syntactic unification

See this answer for more details.

3

What do you mean by “variable aliasing”?

SWI-Prolog could have also output X=3,Y=3 and that’s exactly equivalent to X=Y,Y=3 – there is no way that you can distinguish the two. It’s more compact to write it as X=Y if the result is some large complex term.

Put another way, X=Y means that there is no way to distinguish X and Y. These statements produce the same result:

X=Y, X=3.
X=Y, Y=3.
X=3, X=Y.
Y=X, 3=Y.
4

Thanks, that’s helpful. In the link to the other answer, I found this statement added some clarity for me:

[it] is generally not observable whether two terms are equal because they have been unified or simply because they happen to have the same value.

Again, thanks.