This how-to aims at showing with a short example how a Lisp only function can be accessed from Pilog. It illustrates the last part of the Pilog documentation in https://software-lab.de/doc/ref.html#pilog which says that "Pilog can be called from Lisp and vice versa".

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 a direct translation into Pilog like the following rule:

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

doesn't work, and the query:

(? (underage @X) )

yields to NIL instead of the expected result @X=Kate.

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

In order to embed a Lisp expression in 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, a working translation of the Prolog rule above is:

```      (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 (@X)
(age @X @Y)
(less_than @Y 18) )```

and now:

(? (underage @X) )

yields to:

@X=Kate

which is the expected result.

*This how to is the result of a discussion on the mailing list during november 2016 started here http://www.mail-archive.com/picolisp@software-lab.de/msg06381.html.

https://picolisp.com/wiki/?accesstolispfunctionfrompilog

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