An Annotaed Solution to the Prolog Zebra Problem

November 23, 2025

My solution is the simplified Zebra problem from this SWI-Prolog tutorial.

Here’s the problem statement:

There is a street with three neighbouring houses that all have a different colour, namely red, blue, and green. People of different nationalities live in the different houses and they all have a different pet. Here are some more facts about them:

• The Englishman lives in the red house.
• The jaguar is the pet of the Spanish family.
• The Japanese lives to the right of the snail keeper.
• The snail keeper lives to the left of the blue house.

Who keeps the zebra? Don’t work it out for yourself: define a predicate zebra/1 that tells you the nationality of the owner of the zebra!

% Define a toLeft predicate
% Succeeds if the list contains the sublist H1,H2 at any location
toLeft(H1,H2,[H1,H2|_]).
toLeft(H1,H2,[_|T]):-
  toLeft(H1,H2,T).


zebra(Who):-
  % We have a street of length 3
  length(Street, 3),

  % The street will have an Englishman's red house and a Spaniard with a pet Jaguar
  member(house(english, red, _), Street),
  member(house(spanish, _, jaguar), Street),

  % The Japanese house is to the left of the snail house
  % The blue house is to the left of the snail house
  toLeft(
    house(_,_,snail),
    house(japanese,_,_),
    Street
  ),
  toLeft(
    house(_,_,snail),
    house(_,blue,_),
    Street
  ),

  % We're looking for who owns the Zebra, so create that rule here
  member(house(Who,_,zebra), Street).

« “your system has run out of application memory” on macOS Tahoe | Home | Flatten Nested Lists in Prolog »