Poplog (Prolog)

Implementation of programming language Prolog

Poplog is a free cross-platform compiler created at the University of Sussex which can process several programming languages related to artificial intelligence research. Poplog itself is written in POP-11.

One of the languages implemented in Poplog is Prolog in its classical form. Poplog allows downloading facts and rules database from file and running interactive queries in text mode.



Example for versions Poplog 15.5 (Prolog)

This example consists of two parts — the first part of code should be stored in a file fact.pl placed in working folder of Poplog, and the second one has to be entered manually. [-fact]. downloads facts and rules from this file to current Prolog session (and outputs fact reconsulted to note that download succeeded). Query fact(16,X). tries to find value of X for which this predicate will evaluate to true. The output required in the example is side effect of query evaluation, and the actual result will be X = 20922789888000 ?. This means that if you’re not satisfied with this binding, you can reject it (by entering ;), and the search for better binding will continue.

% fact.pl
fact(X, F) :- 
    ( X=0, F=1; 
      Y is X-1, fact(Y, Z), F is X*Z), 
    write(X), write('! = '), write(F), nl.

% interactive

Fibonacci numbers:

Example for versions Poplog 15.5 (Prolog)

Straightforward recursive implementation is too memory inefficient to be executed in Poplog, so this example shows a more advanced technique — recursion with memoization. An additional predicate memo(Goal) is defined so that the first time Goal is evaluated, the result of its evaluation is added to facts database, and next time it is questioned, it is not re-evaluated but taken as a known fact.

After this predicate fib(N,F) is defined recursively, but each call to fib is wrapped in memo, so for each value of N fib(N,F) is evaluated only once. With such approach printing calculated numbers can be done immediately after their calculation, without extra loop.

% fibonacci.pl
:- dynamic(stored/1).

memo(Goal) :-
    stored(Goal) -> true;
    Goal, assertz(stored(Goal)).

fib(1,1) :- !, write('1, ').
fib(2,1) :- !, write('1, ').
fib(N,F) :-
    N1 is N-1, memo(fib(N1,F1)), 
    N2 is N-2, memo(fib(N2,F2)), 
    F is F1 + F2,
    write(F), write(', ').

% interactive
fib(16,X), write('...'), nl.

Hello, World!:

Example for versions B-Prolog 7.4 #3, Poplog 15.5 (Prolog), gprolog 1.3.0, swipl 5.6.x

This example doesn’t need any facts or rules to be evaluated. The query is executed in interactive mode, and results in the following output:

Hello, World!

First line is the actual output of write predicate, and second line is the result of query evaluation.

Note that replacing single-quotes with double-quotes makes Prolog output the string as an array of ASCII-codes of individual characters:

| ?- write("Hello, World!").


write('Hello, World!'), nl.