Learn Prolog Now! - Chapter 3 - Practical Session - Programming Exercise 3 - Travel routes

lpn-3-practical-3.pl

% Knowledge base of travel routes (provided)
% Assumption: these routes are unidirectional
byCar(auckland,hamilton).
byCar(hamilton,raglan).
byCar(valmont,saarbruecken).
byCar(valmont,metz).

byTrain(metz,frankfurt).
byTrain(saarbruecken,frankfurt).
byTrain(metz,paris).
byTrain(saarbruecken,paris).

byPlane(frankfurt,bangkok).
byPlane(frankfurt,singapore).
byPlane(paris,losAngeles).
byPlane(bangkok,auckland).
byPlane(singapore,auckland).
byPlane(losAngeles,auckland).

% travel/3 - In order to travel from X to Y, what route(s) Z should we take?
travel(X, Y, go(X, Y)) :- byCar(X, Y); byTrain(X, Y); byPlane(X, Y).
travel(X, Y, go(X, Z, go(Z, Y))) :-
  byCar(X, Z), travel(Z, Y, go(Z, Y));
  byTrain(X, Z), travel(Z, Y, go(Z, Y));
  byPlane(X, Z), travel(Z, Y, go(Z, Y)).
travel(X, Y, go(X, Z, go(Z, W, P))) :-
  byCar(X, Z), travel(Z, Y, go(Z, W, P));
  byTrain(X, Z), travel(Z, Y, go(Z, W, P));
  byPlane(X, Z), travel(Z, Y, go(Z, W, P)).