Ernest Friedman-Hill wrote:
(defrule crossover
?e <- (side (name ?ename) {occup == FALSE})
?w <- (side (name ?wname) {occup == TRUE})
?br <- (bridge (name ?bname){occup == FALSE}
(east $east){east == ?e}
(west $west){west == ?w})
=>
(printout t ?bname " " ?ename " " ?wname crlf)
)
Unfortunately these "simple patterns" have confusing semantics, so
they're being redefined somewhat for Jess 7.1. The confusing thing is
that a name like "occup" in the above represents a variable that is
bound automatically to the value of the first slot in which its used.
If you use the same name later in the rule, that variable represents a
unification -- i.e., the same value.
Very well hidden ;-)
One other system I know of lets you write (adopting the Jess syntax)
(defrule crossover
?e <- (side {?e.occup == FALSE}) ; or some way of saying "this"?
?w <- (side {?w.occup == TRUE})
?br <- (bridge {?br.occup == FALSE}
{?br.east == ?e}
{?br.west == ?w})
=>
(printout t ?br.name " " ?e.name " " ?w.name crlf)
But this implies that binding a fact also binds all slots automatically,
and I can't say what this would mean for Jess. (That other system maps a
fact to a C-like struct variable, and therefore the above notation comes
for free.)
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