SWI Prolog description of call/1: "clauses may have variables as subclauses"?

Question

The description of call/1 says:

call(:Goal)

Invoke Goal as a goal. Note that clauses may have variables as subclauses, which is identical to call/1.

I don't understand "clauses may have variables as subclauses".

Can anyone give an example?

Answer 1

Body conversion (7.6.2 Converting a term to the body of a clause) à la ISO core standard requires that variables directly inside control constructs inside a body of a clause are wrapped by call/1.

Here is an example:

?- [user].
and(X,Y) :- X,Y.
^D
?- listing.
and(X,Y) :- call(X), call(Y).

The effect is that calling a ! has no effect, since the cut is confined inside call/1.

This is seen here:

?- [user].
p(a).
p(b).
?- p(X), !.
X = a
?- and(p(X),!).
X = a ;
X = b

Edit 31.12.2019:
A Prolog system might want to add (*->)/2 (soft cut) to those control constructs whose arguments are converted thus amending 7.6.2.

---

From ISO/IEC 13211-1: 1995 Prolog

7.6.2 Converting a term to the body of a clause

A term T can be converted to a goal G which is the body of a clause:

a)

If T is a variable then G is the control construct call (7.8.3), whose argument is T.

b)

If T is a term whose principal functor appears in table 9 then G is the corresponding control construct. If the principal functor of T is call/1 or catch/3 or throw/1 then the arguments of T and G are identical, else if the principal functor of T is (',')/2 or (;)/2 or (->)/2 then each argument of T shall also be converted to a goal.

c)

If T is an atom or compound term whose principal functor FT does not appear in table 9 then G is a predication whose predicate indicator is PT, and the arguments, if any, of T and G are identical.

And

7.8.3 call/1

call(G) is true iff G represents a goal which is true.

When G contains ! as a subgoal, the effect of ! shall not extend outside G.

And

Table 9 on page 37: Principal functors and control constructs

• (',')/2 goal arguments converted
• (;)/2 goal arguments converted
• (->)/2 goal arguments converted
• !/0
• call/1 goal arguments not converted
• true/0
• fail/0
• catch/3 goal arguments not converted
• throw/1