Learn PROLOG with Real Code Examples

Updated Nov 20, 2025

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Code Sample Descriptions

Prolog Counter Simulation

:- dynamic(count/1).
:- dynamic(isDark/1).

count(0).
isDark(false).

updateUI :-
    count(C),
    isDark(D),
    write('Counter: '), write(C), nl,
    write('Theme: '), (D = true -> write('Dark') ; write('Light')), nl.

increment :-
    count(C),
    C1 is C + 1,
    retract(count(C)),
    assert(count(C1)),
    updateUI.

decrement :-
    count(C),
    C1 is C - 1,
    retract(count(C)),
    assert(count(C1)),
    updateUI.

toggleTheme :-
    isDark(D),
    D1 is (D = false -> true ; false),
    retract(isDark(D)),
    assert(isDark(D1)),
    updateUI.

reset :-
    retract(count(_)),
    assert(count(0)),
    updateUI.

% Simulate actions
updateUI,
increment,
increment,
toggleTheme,
decrement,
reset.

Demonstrates a simple counter simulation with theme toggle using Prolog facts and rules.

Let’s Try →

Prolog Simple Addition

add(A,B,Sum) :- Sum is A + B.

% Example
?- add(10,20,S), write(S), nl.

Adds two numbers and prints the result.

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Prolog Factorial

factorial(0,1).
factorial(N,F) :- N > 0, N1 is N-1, factorial(N1,F1), F is N * F1.

% Example
?- factorial(5,F), write(F), nl.

Calculates factorial using recursion.

Let’s Try →

Prolog Fibonacci Sequence

fib(0,0).
fib(1,1).
fib(N,F) :- N > 1, N1 is N-1, N2 is N-2, fib(N1,F1), fib(N2,F2), F is F1 + F2.

% Example
?- fib(10,F), write(F), nl.

Generates Fibonacci numbers recursively.

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Prolog Max of Two Numbers

max(A,B,A) :- A >= B.
max(A,B,B) :- B > A.

% Example
?- max(10,20,M), write(M), nl.

Finds the maximum of two numbers.

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Prolog List Sum

sum_list([],0).
sum_list([H|T],Sum) :- sum_list(T,SumT), Sum is H + SumT.

% Example
?- sum_list([1,2,3,4,5],S), write(S), nl.

Sums elements of a list recursively.

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Prolog Even Numbers Filter

even(X) :- 0 is X mod 2.
print_even([]).
print_even([H|T]) :- even(H), write(H), nl, print_even(T).
print_even([H|T]) :- \+ even(H), print_even(T).

% Example
?- print_even([1,2,3,4,5]).

Prints even numbers from a list.

Let’s Try →

Prolog Conditional Counter Increment

:- dynamic(count/1).
count(3).
increment_if_less_than_5 :- count(C), C < 5 -> C1 is C + 1, retract(count(C)), assert(count(C1)); true.

% Example
?- increment_if_less_than_5, count(C), write(C), nl.

Increments counter only if less than 5.

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Prolog Resettable Counter

:- dynamic(count/1).
count(0).
increment :- count(C), C1 is C + 1, retract(count(C)), assert(count(C1)).
reset :- retract(count(_)), assert(count(0)).

% Example
?- increment, increment, count(C), write(C), nl, reset, count(C2), write(C2), nl.

Counter that can increment and reset.

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Prolog Theme Toggle Only

:- dynamic(isDark/1).
isDark(false).
toggle_theme :- isDark(D), D1 is (D = false -> true ; false), retract(isDark(D)), assert(isDark(D1)).

% Example
?- write('Initial Theme: '), isDark(D), write(D), nl, toggle_theme, isDark(D1), write('Toggled Theme: '), write(D1), nl.

Toggles theme multiple times.

Let’s Try →

Code Sample Descriptions

Prolog Counter Simulation

:- dynamic(count/1).
:- dynamic(isDark/1).

count(0).
isDark(false).

updateUI :-
    count(C),
    isDark(D),
    write('Counter: '), write(C), nl,
    write('Theme: '), (D = true -> write('Dark') ; write('Light')), nl.

increment :-
    count(C),
    C1 is C + 1,
    retract(count(C)),
    assert(count(C1)),
    updateUI.

decrement :-
    count(C),
    C1 is C - 1,
    retract(count(C)),
    assert(count(C1)),
    updateUI.

toggleTheme :-
    isDark(D),
    D1 is (D = false -> true ; false),
    retract(isDark(D)),
    assert(isDark(D1)),
    updateUI.

reset :-
    retract(count(_)),
    assert(count(0)),
    updateUI.

% Simulate actions
updateUI,
increment,
increment,
toggleTheme,
decrement,
reset.

Demonstrates a simple counter simulation with theme toggle using Prolog facts and rules.

Let’s Try →

Prolog Simple Addition

add(A,B,Sum) :- Sum is A + B.

% Example
?- add(10,20,S), write(S), nl.

Adds two numbers and prints the result.

Let’s Try →

Prolog Factorial

factorial(0,1).
factorial(N,F) :- N > 0, N1 is N-1, factorial(N1,F1), F is N * F1.

% Example
?- factorial(5,F), write(F), nl.

Calculates factorial using recursion.

Let’s Try →

Prolog Fibonacci Sequence

fib(0,0).
fib(1,1).
fib(N,F) :- N > 1, N1 is N-1, N2 is N-2, fib(N1,F1), fib(N2,F2), F is F1 + F2.

% Example
?- fib(10,F), write(F), nl.

Generates Fibonacci numbers recursively.

Let’s Try →

Prolog Max of Two Numbers

max(A,B,A) :- A >= B.
max(A,B,B) :- B > A.

% Example
?- max(10,20,M), write(M), nl.

Finds the maximum of two numbers.

Let’s Try →

Prolog List Sum

sum_list([],0).
sum_list([H|T],Sum) :- sum_list(T,SumT), Sum is H + SumT.

% Example
?- sum_list([1,2,3,4,5],S), write(S), nl.

Sums elements of a list recursively.

Let’s Try →

Prolog Even Numbers Filter

even(X) :- 0 is X mod 2.
print_even([]).
print_even([H|T]) :- even(H), write(H), nl, print_even(T).
print_even([H|T]) :- \+ even(H), print_even(T).

% Example
?- print_even([1,2,3,4,5]).

Prints even numbers from a list.

Let’s Try →

Prolog Conditional Counter Increment

:- dynamic(count/1).
count(3).
increment_if_less_than_5 :- count(C), C < 5 -> C1 is C + 1, retract(count(C)), assert(count(C1)); true.

% Example
?- increment_if_less_than_5, count(C), write(C), nl.

Increments counter only if less than 5.

Let’s Try →

Prolog Resettable Counter

:- dynamic(count/1).
count(0).
increment :- count(C), C1 is C + 1, retract(count(C)), assert(count(C1)).
reset :- retract(count(_)), assert(count(0)).

% Example
?- increment, increment, count(C), write(C), nl, reset, count(C2), write(C2), nl.

Counter that can increment and reset.

Let’s Try →

Prolog Theme Toggle Only

:- dynamic(isDark/1).
isDark(false).
toggle_theme :- isDark(D), D1 is (D = false -> true ; false), retract(isDark(D)), assert(isDark(D1)).

% Example
?- write('Initial Theme: '), isDark(D), write(D), nl, toggle_theme, isDark(D1), write('Toggled Theme: '), write(D1), nl.

Toggles theme multiple times.

Let’s Try →