not/1 is not a built-in in gprolog. Instead, use \+ / 1. This means
"fails to solve", the negation-as-failure predicate.
Many prologs avoid the not/1 built-in because it is not possible to
implement it in the fully general case and having the built-in would be
misleading. You could define a not/1 predicate as:
not(Goal) :-
\+ call(Goal).
The message "uncaught exception:
error(existence_error(procedure,not/1),action/2)" is gprolog's graceful
:-) way of telling you that not/1 is an undefined procedure (predicate)
encountered while solving an action/2 clause.
The behavior you were looking for, a missing predicate definition
causing a simple failure, can be caused by setting the prolog flag
'unknown' to 'fail'. Solve the following goal on the command line of
gprolog:
set_prolog_flag(unknown, fail).
HTH,
Lindsey Spratt
On Aug 30, 2005, at 7:24 PM, Cliff Bender wrote:
Hi. I'm new to prolog, but am trying to learn the language to fulfill
a prereq for a grad school program. I'm currently using Niel C. Rowe's
"Artificial Intelligence Through Prolog" for reference and sample
programs. I've typed out the traffic program on pages 61-62:
/*--------Rules for arrow lights--------*/
/*A*/ action(car, stop) :- light(yellow_arrow, Direction),
safe_stop_possible.
/*D,E*/ action(car, yield_and_leftturn) :- light(yellow_arrow, left),
not(safe_stop_possible).
[snip program details]
given the facts:
safe_stop_possible.
light(yellow,steady).
light(green_arrow,left).
the query action(car,X) returns:
X = yield_and_go ? ;
uncaught exception: error(existence_error(procedure,not/1),action/2)
if I don't put safe_stop_possible in as a fact, it blows up as well.
can anyone help? this is leaving me stumped. i thought not was a
built-in predicate? and if a fact isn't declared/asserted, that just
means that querying it would return false, correct?
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