Prolog Cheat Sheet

Core Prolog concepts covering facts, rules, queries, list operations, and built-in predicates for logic programming.

2 PagesAdvancedApr 8, 2026

Facts & Rules

Defining facts and a derived rule.

prolog

% Factsparent(tom, bob).parent(bob, ann).parent(bob, pat).% Rule: X is a grandparent of Z if X is a parent of Y% and Y is a parent of Zgrandparent(X, Z) :- parent(X, Y), parent(Y, Z).% Rule with a not-equal checksibling(X, Y) :- parent(P, X), parent(P, Y), X \= Y.

Queries

Asking questions against the knowledge base.

prolog

?- parent(tom, bob).true.?- grandparent(tom, X).X = ann ;X = pat.?- parent(X, Y), parent(Y, ann).X = tom,Y = bob.

List Operations

Working with Prolog lists.

[H|T]- Head/tail list decomposition pattern
length(List, N)- N unifies with the length of List
append(L1, L2, L3)- L3 is L1 concatenated with L2
member(X, List)- True if X occurs in List
reverse(L1, L2)- L2 is L1 reversed
nth0(Index, List, Elem)- Elem is the element at the 0-based Index

Built-in Predicates

Commonly used core predicates.

=- Unification of two terms
\=- True if two terms do not unify
is/2- Arithmetic evaluation, e.g. X is 2 + 3
!- Cut; commits to choices made so far and prunes backtracking
findall(Template, Goal, List)- Collects all solutions of Goal into List
write/1, nl/0- Print a term / print a newline

Recursion Example

A classic recursive factorial predicate.

prolog

factorial(0, 1).factorial(N, F) :-    N > 0,    N1 is N - 1,    factorial(N1, F1),    F is N * F1.?- factorial(5, X).X = 120.

Pro Tip

Use the cut (!) sparingly and only after you understand its effect on backtracking — an overused cut silently discards valid solutions and makes predicates hard to reuse.

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