Prolog Programming Language Cheatsheet

Prolog, short for “Programming in Logic,” is a declarative programming language designed for symbolic reasoning and manipulation. It is particularly well-suited for tasks involving rule-based systems, artificial intelligence, and knowledge representation. If you’re diving into Prolog or need a quick reference guide, this cheatsheet will help you get started and navigate the essentials of the language.

1. Basic Syntax:

  • Facts: likes(john, pizza). friend(jane, alice).
  • Rules: mortal(X) :- human(X). human(socrates).
  • Queries:
    prolog ?- likes(john, pizza). ?- mortal(socrates).

2. Variables:

  • Variables in Prolog begin with an uppercase letter or an underscore.
   ?- likes(john, X).

3. Lists:

  • Lists are a fundamental data structure in Prolog.
   colors([red, green, blue]).
  • Accessing elements in a list:
   ?- colors(Colors), nth0(1, Colors, SecondColor).

4. Predicates:

  • Built-in predicates are essential for manipulating data and performing operations.
   ?- length([1, 2, 3], Len).
   ?- append([1, 2], [3, 4], Result).

5. Arithmetic Operations:

   ?- X is 3 + 4.

6. Cut Operator (!):

  • Cuts off backtracking, committing to the current choice point.
   likes(john, pizza) :- !.
   likes(john, sushi).

7. Recursion:

  • Prolog excels in recursive definitions.
   factorial(0, 1).
   factorial(N, Result) :-
       N > 0,
       Prev is N - 1,
       factorial(Prev, PrevResult),
       Result is N * PrevResult.

8. Conjunction and Disjunction:

  • Conjunction (,): ancestor(X, Y) :- parent(X, Z), ancestor(Z, Y).
  • Disjunction (;):
    prolog color(X, green) :- leaf(X). color(X, brown) :- trunk(X).

9. Dynamic Predicates:

  • Modify facts or rules at runtime.
   asserta(fact(X)) :- !, asserta(X).

10. Debugging:

  • Use trace. to enable tracing for better debugging.
   ?- trace.


Prolog’s unique paradigm and elegant syntax make it a powerful tool for tasks involving logical reasoning and knowledge representation. This cheatsheet covers the basics, providing a foundation for exploring more advanced features and applications in Prolog programming. As you delve deeper into the language, experimenting with different predicates, rules, and constructs will enhance your understanding and proficiency.

You can refer to the official prolog documentation for more in-depth information on each topic.

1. What makes Prolog different from other programming languages?

Prolog stands out as a declarative programming language designed for symbolic reasoning. Unlike imperative languages, Prolog focuses on expressing relationships and rules rather than explicit sequences of instructions. Its unique approach to problem-solving, using a formal logic foundation, makes it particularly well-suited for tasks in artificial intelligence, expert systems, and knowledge representation.

2. How does backtracking work in Prolog, and why is it important?

Backtracking is a key feature in Prolog that enables the system to explore multiple solutions to a problem. When a query is made, Prolog will search for a solution, and if it encounters a choice point (multiple possibilities), it can backtrack and explore alternative paths. This mechanism is essential for exploring different logical branches and finding all possible solutions to a given problem.

3. Can Prolog be used for practical software development, or is it limited to specific domains?

While Prolog is not as commonly used in mainstream software development as languages like Java or Python, it has found significant applications in specific domains. Prolog excels in rule-based systems, natural language processing, and expert systems. Its expressiveness in representing and solving logical problems makes it a valuable tool in certain niches, although it might not be the first choice for general-purpose programming.

4. Are there limitations to Prolog, and what challenges might developers face?

Prolog’s strengths lie in certain problem domains, but it may not be the best choice for every type of application. Developers might face challenges when dealing with efficiency concerns, especially in large-scale systems. Understanding and optimizing the execution model, as well as managing backtracking efficiently, are areas where developers may encounter hurdles.

5. How does Prolog handle input and output?

Prolog primarily focuses on symbolic reasoning and logic, and its input/output mechanisms differ from traditional imperative languages. Input in Prolog is often handled through queries, where users pose questions to the system. Output is the result of these queries, displaying the solutions to the specified logical problems. Prolog doesn’t follow the conventional input/output paradigm found in languages like C or Python, emphasizing its unique approach to problem-solving.