Prolog Examples

A small collection of Prolog programs illustrating facts, rules, and constraint solving.

Prerequisites

Install SWI-Prolog (version 10+):

# Windows (winget)
winget install SWI-Prolog.SWI-Prolog

# macOS
brew install swi-prolog

# Ubuntu/Debian
sudo apt install swi-prolog

After installation, swipl will be on your PATH (you may need to restart your terminal).

Running Programs

Interactive REPL

Load a file into the interactive interpreter and type queries at the ?- prompt:

swipl friends.pl

?- friend(wallace, grommit).
true.

?- halt.

Script / Batch Mode

Use -g to run a goal non-interactively and -t halt to exit when done:

swipl -q -l friends.pl -g "friend(wallace, grommit), write(true)" -t halt

The -q flag suppresses the startup banner.

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Files

friends.pl — Facts, Rules, and Negation

Defines likes/2 facts and a friend/2 rule using negation-as-failure (\+).

likes(wallace, cheese).
likes(grommit, cheese).
likes(wendolene, sheep).

friend(X, Y) :- \+(X = Y), likes(X, Z), likes(Y, Z).

Interactive queries:

?- friend(wallace, grommit).
true.

?- friend(grommit, wallace).
true.

?- friend(wallace, wendolene).
false.

?- likes(wallace, What).
What = cheese.

Note: Querying friend(X, Y). with unbound variables returns no results because \+(X = Y) fails when both X and Y are uninstantiated (they unify with each other). Bind the variables through likes/2 first if you want to enumerate all pairs:

?- likes(X, _), likes(Y, _), friend(X, Y).

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food.pl — Chained Rules

Demonstrates multi-step inference: food_type + flavor facts → food_flavor rule.

food_type(velveeta, cheese).  food_flavor(X, Y) :- food_type(X, Z), flavor(Y, Z).
flavor(savory, cheese).
flavor(savory, meat).
flavor(sweet, dessert).
flavor(sweet, soda).

Interactive queries:

?- food_flavor(spam, What).
What = savory.

?- food_flavor(twinkie, What).
What = sweet.

?- food_flavor(What, sweet).
What = jolt ;
What = twinkie.

?- food_flavor(X, Y).
X = velveeta, Y = savory ;
X = spam,     Y = savory ;
X = sausage,  Y = savory ;
X = jolt,     Y = sweet  ;
X = twinkie,  Y = sweet.

ritz has no flavor because flavor(_, cracker) is not defined in the knowledge base.

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map.pl — Constraint / Map Coloring

Uses Prolog's search to find a valid 3-coloring of five neighboring US states.

different(red, green). different(red, blue).
different(green, red). different(green, blue).
different(blue, red).  different(blue, green).

Interactive queries:

?- coloring(Alabama, Mississippi, Georgia, Tennessee, Florida).
Alabama = blue,
Mississippi = red,
Georgia = red,
Tennessee = green,
Florida = green ;
...

Press ; to see the next solution, or . to stop.

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ohmy.pl — Unification and Pattern Matching

Illustrates unification (=) and generating combinations.

cat(lion).  cat(tiger).

dorothy(X, Y, Z) :- X = lion, Y = tiger, Z = bear.
twin_cats(X, Y) :- cat(X), cat(Y).

Interactive queries:

?- dorothy(X, Y, Z).
X = lion,
Y = tiger,
Z = bear.

?- twin_cats(X, Y).
X = lion, Y = lion ;
X = lion, Y = tiger ;
X = tiger, Y = lion ;
X = tiger, Y = tiger.

?- cat(lion).
true.