On Mar 22, 2007, at 12:33 PM, Shi Paul wrote:
Huh? It is a bit out of my expectation, I thought it should be very
simple because of my naiveness. So does the same theory applys to
the internal implementation of Jess' assert function?
Imagine saying "I'd like to write a new Java keyword 'foo' which is
the same as 'class' but a little different. 'foo' should be followed
by a class definition." Silly, right? It's pretty obvious it can't be
done without modifying the Java compiler.
What you're asking is the equivalent thing in Jess. "assert" is
special just like "class" is special; the Jess compiler knows about
"assert" just like the Java compiler knows about "class". You can't
write a function in Jess that behaves like 'assert' anymore than you
can write a function in Java which behaves like 'class'.
Now, as it happens, there is an ugly hack which will let you do what
you want in Jess, and I described it below. But it is just a hack,
not something I'd expect anyone to do in production code. If you
really want to extend the Jess language in this way, then you need
to modify the parser itself.
Thanks,
Paul
From: "Ernest Friedman-Hill" <[EMAIL PROTECTED]>
Reply-To: [email protected]
To: [email protected]
Subject: Re: JESS: how do I write my assert function?
Date: Thu, 22 Mar 2007 12:18:49 -0400
On Mar 22, 2007, at 11:42 AM, Shi Paul wrote:
Hi there,
I'm a novice, so pls forgive me if I ask something stupid. I'd
like to add an user function which is doing exactly the same
thing as (assert (abc(slot1 ?s1)(slot2 ?s2))). The parameter for
the fact I got within that function (vv.get(1)) is an instanceof
FuncallValue, how could I turn that parameter into a Fact or
maybe a String, so that I can use engine.assertFact(xx) or
engine.assertString(xx)?
Every set of parentheses in Jess code denotes a function call...
except when they don't. These special cases where they don't are
traditionally (in Lisp) called "special forms." Special forms are
handled by the parser itself. "defrule" is a special form, as
are other constructs like 'deftemplate', 'defquery', etc.
"assert" is also a special form, because its arguments are facts,
not function calls. In other words, you can't do what you're
asking, because the parser is going to have already decided your
fact, above, is a call to function "abc" with calls to "slot1"
and "slot2" as arguments. Because Jess does extremely late
binding, the fact that these functions don't exist won't be
determined until you invoke the functions by calling
Value.resolveValue() or any of the xxxValue() functions that
return a scalar like symbolValue() or intValue(). Strictly, the
only way to do what you want is by modifying the parser itself
(the class jess.Jesp).
But if you're willing to live with the fact that the parser won't
error-check your facts (because it doesn't know they are facts) I
can actually describe a hack that *would* work. The argument to
your function will look like a FuncallValue, and you can call
funcallValue () to get a jess.Funcall. This latter class is a
subclass of ValueVector. The first element in the ValueVector is
the name of the function, and the later elements are the
arguments. So what you'd have to do is take the name of the top-
level Funcall, and use that as the "head" of the fact; then
collect all the nested Funcalls; each of their names will
actually be a slot name, and their first (and only) argument will
be the slot value. Given these values, you can build a jess.Fact
yourself and assert it.
Gross, huh?
---------------------------------------------------------
Ernest Friedman-Hill
Advanced Software Research Phone: (925) 294-2154
Sandia National Labs FAX: (925) 294-2234
PO Box 969, MS 9012 [EMAIL PROTECTED]
Livermore, CA 94550 http://www.jessrules.com
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Ernest Friedman-Hill
Advanced Software Research Phone: (925) 294-2154
Sandia National Labs FAX: (925) 294-2234
PO Box 969, MS 9012 [EMAIL PROTECTED]
Livermore, CA 94550 http://www.jessrules.com
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