Learn Prolog Now! - Chapter 6 - Exercise 6.6 - Who owns the zebra? (Simplified version of Einstein's Riddle)

lpn-6-6p6.pl

% Learn Prolog Now! - Chapter 6 - Exercise 6.6 - Who owns the zebra?
% A simplified version of Einstein's Riddle
% Problem Statement:
% There is a street with three neighboring houses that all have a different color, 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?  zebra/1 should tell us who this person is.

% Formulation of the problem statement via the predicate zebra/1
% Heavily inspired by this answer from StackOverflow (to the actual Einstein's Riddle): https://stackoverflow.com/questions/318888/solving-who-owns-the-zebra-programmatically/8270393#8270393
% Correctness of solution not guaranteed
zebra(X) :- % X owns the zebra if ...
  Houses = [_, _, _], % There are 3 neighboring houses, where each house will be represented by house(Color, Nationality, Pet) in the following statements
  member(house(red, english, _), Houses), % The Englishman lives in the red house
  member(house(_, spanish, jaguar), Houses), % The jaguar is the pet of the Spanish family
  nextto(house(_, _, snail), house(_, japanese, _), Houses), % The Japanese lives to the right of the snail keeper
  nextto(house(_, _, snail), house(blue, _, _), Houses), % The snail keeper lives to the left of the blue house
  member(house(_, X, zebra), Houses). % Who (X) keeps the zebra?