Array Univ var ([x,y]=..T) - prolog

Question

I saw a question asking what does [a,b,c]=..L. return. When testing this I saw that it returns: L = ['.', a, [b, c]].

I can't understand why this happens, I was unable to understand the nature of Univ from the documentation. Understanding this will help me understand Univ.

Answer 1

One way to learn more about terms is to use write_canonical/1 in a conforming Prolog system.

For example, with GNU Prolog, we obtain:

| ?- write_canonical([x,y]).
'.'(x,'.'(y,[]))

This shows:

1. the primary functor in this term is '.', with arity 2
2. the first argument is x
3. the second argument is '.'(y, []), which is the list [y]

This explains why (=..)/2 yields:

| ?- [x,y] =.. Ls.

Ls = ['.',x,[y]]

and also your other example.