How do I assert a fact in a CLIPS deffunction? - clips

Question

I originally attempted using the modify function but it doesn't do anything and just prints false, I don't know what I am doing wrong.
I used
(modify ?tv (v ?x))
it didn't work.
I then used
(retract ?tv)
(assert (v ?x))
instead, which worked.
But I don't want to type that out every time I want to modify a fact, so I made a deffunction to do it for me, but
(deffunction modfact(?index ?factname ?factvalue)
(retract ?index)
(assert (?factname ?factvalue))
)
in this it gives a syntax error of:
[PRNTUTIL2] Syntax Error: Check appropriate syntax for first field of a RHS pattern.
ERROR:
(deffunction MAIN::modfact
(?index ?factname ?factvalue)
(retract ?index)
(assert (?factname
Which seems to me that its saying that I can't actually make this function because I can't assert a fact with the value of the variable. How can I get this to work?

Answer 1

Modify only works with facts that have an associated deftemplate defined with slots:
CLIPS>
(deftemplate task
(slot id)
(slot completed))
CLIPS> (watch facts)
CLIPS> (assert (task (id x) (completed no)))
==> f-1 (task (id x) (completed no))
<Fact-1>
CLIPS>
(defrule modit
?f <- (task (completed ~yes))
=>
(modify ?f (completed yes)))
CLIPS> (run)
<== f-1 (task (id x) (completed no))
==> f-2 (task (id x) (completed yes))
CLIPS>
When using the assert command, the first field of the fact must be a symbol. If you must get around this restriction you can use the str-assert function.
CLIPS>
(deffunction modfact (?index ?factname ?factvalue)
(retract ?index)
(str-assert (str-cat "(" ?factname " " ?factvalue ")")))
CLIPS> (assert (v 3))
==> f-3 (v 3)
<Fact-3>
CLIPS> (modfact 3 v 4)
<== f-3 (v 3)
==> f-4 (v 4)
<Fact-4>
CLIPS>