I am reading the jess manual, exactly in 6.12. The 'test' conditional element. I copied exactly the example for try running this code, but didn´t work at all.
NOTE: I have run another examples and work fine.
(deftemplate person (slot age))
(defrule example-8
(test (eq 4 (+ 2 2)))
=>
(printout t "2 + 2 is 4!" crlf))
My result is nothing.. don´t show me any message, don´t return any error message. I really want to understand it. Please help me on this.
Many rules — and this is one — require the “initial-fact” asserted by “reset” to operate. The manual explains in more detail, but just as a best practice, you generally need to call (reset) before asserting any facts and calling (run).
Related
The SICStus Prolog manual page on mutable terms states that:
[...] the effect of unifying two mutables is undefined.
Then, why does create_mutable(data,x) fail?
Shouldn't that rather raise an uninstantiation_error?
I cannot think of a situation when above case is not an unintentional programming error (X vs x)... please help!
The short answer to "Why does create_mutable/2 not throw an exception when output unification fails?" is just: Because this was how it was done when the feature was added to SICStus Prolog, and no one has made a strong case for changing this.
One important "difference between the stream created by open/4 and the mutable term created by create_mutable/2" is that open/4 has side-effects that are not undone if the output-unification of the call to open/4 fails.
In this sense, create_mutable/2 is somewhat more like is/2 which also just quietly fails if the output argument is some non-numeric non-variable term, e.g. in x is 3+4. This seems to be the common, and traditional, way of handling output arguments in Prolog.
I agree that a non-variable as second argument is most likely a programming error. The next version of the SICStus IDE, SPIDER, will warn for this (as it already does for is/2).
None of this, nor the example in the question, seems directly related to the cited documentation "[...] the effect of unifying two mutables [...]".
I'm working on some lisp code that munges a CVS repository in order to get it into shape for a git conversion. As such, when something goes wrong it might not be a bug in my code but might instead be an inaccuracy in the input data which needs fixing.
Deep down in the bowels of my code, I'll have (say) some function that merges two date ranges by taking an intersection. If that intersection turns out to be empty, I'll raise an error. This is all good, but my "merge dates" function is missing lots of the information that the user (me!) needs in order to figure out what went wrong. For example, which CVS master file ("foo,v") was I working on? What branch was I thinking about? Etc. etc.
My partial solution to this problem is to handle and re-raise the error. For example, this sort of code:
(handler-case
(do-something)
(unclear-graft-point (c)
(setf (slot-value c 'master) master)
(setf (slot-value c 'branch) branch)
(error c)))
which sets some extra slots with useful info before re-raising the error.
The report function for the condition then checks whether those slots have been set and, if so, will use them to give a more helpful error message.
Unfortunately, the backtrace that I get stops at the top-level re-raise. That makes sense: it's the code I wrote. But it's not really what I want...
Is there a way in Common Lisp to "annotate" a condition as it hurtles past, without re-raising it and, hence, partially unwinding the stack without reporting that in the back trace?
I gave a reasonable amount of background info about what I'm doing in the hope that, if this isn't possible, someone will say "Ah, what you should do in this situation is...": am I just going about this the wrong way?
Yes, there is a Common Lisp technology applicapble to your case - it is called HANDLER-BIND. You can find a detailed explanation and use cases for it alongside other condition-handling machinery in Peter Seibel's Practical Common Lisp Chapter 19. Conditions and Restarts.
I saw such code in a paper:
(import com.psy.entity.Record)
Can this statement import a Java class?
(deftemplate Record(declare(from-class Record)))
I don't understand this statement, how to use the declare statement, is there a function in Jess named from-class, I think I have never seen such function in the book of Jess in Action.
(defrule show-test-result-20
?0<-(Record {score<=30}))
I think it's better to do this test by the test conditional element, is this code right?
"Can...class?" -> Yes, that's why you have the import statement. See the Jess manual.
(deftemplate Record (declare (from-class Record)))
This declares a fact type Record as a shadow fact for class `Record*. This is discussed and explained in the Jess manual.
"I think...?" -> No, the rule is perfectly all right and preferable.
This is for a homework assignment, but I am having a really hard time finding good jess information on the web. I'm trying to use a multislot to to solve a problem, but I can't find anything on how to match the different values. I have this:
(deftemplate patient (slot name)(multislot symptoms))
(deffacts init
(patient (name john) (symptoms very-high-fever cough)))
How can I match the left hand side for just a very-high-fever? This works if I know that the very-high-fever is the first symptom, but I can't be sure of that, so I need to be able to match if a very-high-fever is either symptom.
(defrule high-fever
(patient (name ?n)(symptoms very-high-fever ?))
=>
(printout t ?n " has a high fever." crlf))
I've tried various combinations of field constraints, but I can't seem to get it right and nothing online is giving me any clues.
Thanks.
Use a blank multifield before and after the item you want to match; they match zero or more items. So, something like
(patient (name ?n) (symptoms $? very-high-fever $?))
Will match any patient with the very-high-fever symptom in any position.
can i somehow find all functions/macros that take a specific type of parameter ?
for example, what function accepts a Namespace object as parameter ?
(this is because i can create a namespace and store it in a var, but i don't know where i could use that var; what function might i pass that var to ?)
here is some code:
user=> (def working-namespace (create-ns 'my-namespace))
#'user/working-namespace
;i created a namspace and want to use it later
user=> (class working-namespace)
clojure.lang.Namespace
; out of curiosity i found out that "working-namespace" is a Namespace object
user=> (ns working-namespace)
nil
working-namespace=>
; but how do i switch to it ? this didn't do what i wanted...
user=> (refer working-namespace)
java.lang.ClassCastException: clojure.lang.Namespace cannot be cast to clojure.lang.Symbol (NO_SOURCE_FILE:0)
; this did not work either as my expectations
user=> (the-ns working-namespace)
#<Namespace my-namespace>
user=> (class (the-ns working-namespace))
clojure.lang.Namespace
; great, this gave me the same thing, a Namespace
hence the question: how do i use it dynamically (that's why i had put my namespace into a var) ? how do i get something useful for me from a var that points to a namespace ?
i can try look around for functions that make use of a Namespace object or that convert it to something else. i did and only found "intern". searching by hand not seems not that promising
what if i have this problem a million time ? is there an automated way to get me what i'm looking for without having to ask around each time ?
In Clojure 1.2 and previous function arguments dont have types. every function argument is an object. So the question really becomes "how do i find functions that will cast the object I pass them into this type. so searching for type hints will find some of them, though it wont get you everything. I wish it where more possible to answer this in general.
starting with 1.3 (current dev branch 9/2010) function paramerters and return types can have a defined type and will be passed/returned as that type instead of being cast to object and then cast on the other side. This drops one of the zeros from the exacution time of numerical functions with the important limitation that it only works for :static functions and only with direct calls (ie: not through map/reduce/filter/etc.) There is not a lot published on this change yet though it has the important breaking change that integers are no longer boxed by default and integer (actually Long) overflow throws an exception. you can read more here
(defn ^:static fib ^long [^long n]
(if (<= n 1)
1
(+ (fib (dec n)) (fib (- n 2)))))
so after 1.3 is released and widely adopted you will see code with more commonly defined types because they will offer a big speed benefit and then you will be able to find more functions by argument type though still not all of them.
At the same lecture where I learned about function argument types, Rich mentioned plans in the distant Clojure future (after 'Clojure in Clojure') about better support for exposing the compiler internals to tools such as IDEs. So there is hope that someday you will get a real answer to this question.
Dynamic languages make this slightly more difficult in practice and a lot harder in theory.
You already got a good answer from Arthur, so I'll only answer the "how do i get something useful for me from a var that points to a namespace ?". From (doc ns), note that it says unevaluated:
user=> (doc ns)
-------------------------
clojure.core/ns
([name docstring? attr-map? references*])
Macro
Sets *ns* to the namespace named by name (unevaluated), creating it
Now there's something you could do with in-ns if you want (the whole namespace object -> string -> symbol conversion is probably stupid, but enough to illustrate my point):
user=> (in-ns (symbol (str working-namespace)))
#<Namespace my-namespace>
my-namespace=>
I don't think you can do that without a lot of hackery in a dynamic language. If you want to now what function that take namespaces look at the documentation of the namespace stuff.
For example cleaning namespaces or reload them.
You wrote:
user=> (ns working-namespace)
nil
working-namespace=>
; but how do i switch to it ? this didn't do what i wanted...
But you did switch to the working-namespace namespace (that's why the prompt changed), so I'm not clear as to what "you wanted".
As I noted earlier, you need to present the ultimate problem are you trying to solve. It's entirely likely that messing with namespace objects won't be the solution.