call/1

Module: builtins

call/1 — calls a goal

ISO Standard Predicate

FORMS

call(Goal)

Goal

DESCRIPTION

If Goal is instantiated to a structured term or atom which would be acceptable as the body of a clause, the goal call(Goal) is executed exactly as if that term appeared textually in place of the expression call(Goal).

EXAMPLES

The following example illustrates the use of call/1; first we consult user to input some clauses (note that a newline must be input after each line, and that ^D indicates typing control-D to close user):

?- [user].
jim :- printf("Hello this is Jim Rockford.\n"),
printf("Please leave your name and number,\n"),
printf("and I'll get back to you.\n").
^D
yes.

?- call(jim).
Hello this is Jim Rockford.
Please leave your name and number,
and I'll get back to you.
yes.

The following example shows how an instantiated variable can be used to run a goal:

?- Goal=write(Message), Message='WrongWay!', Goal, nl.
WrongWay!

Goal=write('WrongWay!')
Message='WrongWay!'

yes.

Note that the last two lines Goal=... and Message=.. are the Prolog interpreter providing answer values for the variables in the successful query.

ERRORS

If Goal is an uninstantiated variable or a number, call/1 will fail.

SEE ALSO