Hi Alex,

I've just created an account on the wiki however I think I can't add something in the reference part. I think that this how-to could fit very well here:


to illustrate the last part which starts with " Pilog can be called from Lisp and vice versa:"

After the last sentence and just before the horizontal rule, it could be word like this:


To illustrate this, let's say that you have those two facts in a Pilog database:

    (be age (Paul 19) )
    (be age (Kate 17) )

and that you want to find the person under 18.

In full Prolog you may have written something like this:

    underage(X) :- age(X,Y), Y < 18.

however in Pilog the following rule:

    (be underage (@X)
      (age @X @Y)
      (< @Y 18) )

won't work and the query:

    (? (underage @X) )

will yield to 'NIL' instead of the expected result '@X=Kate' .

The reason is that '<' (less than) is not Pilog function but only a Lisp one in PicoLisp.

In order to embed a Lisp expression in a Pilog, you must use '^' operator. It causes the rest of the expression to be taken as Lisp. Then, inside the Lisp code you can in turn access Pilog-bindings with the '->' function.

Hence, in our case the Prolog rule above translates as:

    (be underage (@X)
      (age @X @Y)
      (^ @ (< (-> @Y) 18)) )

In '(^ @ (< (-> @Y) 18))', '@' is an anonymous variable used to get the result. If you need to access the result you can bind it to a defined variable like in '(^ @B (+ (-> @A) 7))' where '@B' is now bound to '@A + 7'.

You may prefer to define your own Pilog predicate in this particular case. Let's say that to avoid confusion, you want to create a Pilog predicate call 'less_than' to mimic the Lisp function '<':

    (be less_than (@A @B)
      (^ @ (< (-> @A) (-> @B) )))

Then the Pilog rule becomes:

    (be underage_1 (@X)
      (age @X @Y)
      (less_than @Y 18) )

and now:

    (? (underage @X) )

yields to:


which is the expected result.


Hope this helps.


Le 13/11/2016 à 15:47, Alexander Burger a écrit :
You can't be too young, I think. You could write it, and perhaps others
may improve it;)

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