Prolog: Call by pattern

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Many imperative programming languages support the following for communication of actual parameters to formal parameters.

call by value - pass a copy (changing copy does not change original)
call by reference - pass a reference (original can be changed)

In Prolog, call by reference is achieved by passing an unassigned logical variable (that can only be assigned once, backtracking undoes the assignment).

For lack of a more descriptive term, Prolog has what can be described as a call by pattern (matching).

Below are some examples to illustrate the concept. Here is the Prolog code.

prints([]):- nl.
prints([X|Rest]):- print(X), prints(Rest).

run(X):-
	prints(['\n']),
	prints(['X: ',X,'\n']),
	E is X,
	prints(['E: ',E,'\n']).

go:-
	run(2 + 3 * 5),
	run('+'(2, '*'(3, 5))),

	run((2 + 3) * 5),
	run('*'('+'(2, 3),5)).

:- solve(go).

Here are the calls.

run(2 + 3 * 5), run('+'(2, '*'(3, 5))),

Here is the output of the Prolog code.

\n
X: '+'(2,'*'(3,5))\n
E: 17\n
\n
X: '+'(2,'*'(3,5))\n
E: 17\n
\n
X: '*'('+'(2,3),5)\n
E: 25\n
\n
X: '*'('+'(2,3),5)\n
E: 25\n

What appears to be an arithmetic expression

2 + 3 * 5

is actually the following term structure.

'+'(2, '*'(3, 5))

The single quotes are used to represent a symbol (or symbols) as an atom.

What appears to be an arithmetic expression

(2 + 3) * 5

is actually the following term structure.

'*'(+'(2, 3),5)

The single quotes are used to represent a symbol (or symbols) as an atom.