Learn Prolog Now! - Chapter 3 - Practical Session - Programming Exercise 2 - Travelling

lpn-3-practical-2.pl

% Knowledge base of travel information (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/2 - Can we travel from X to Y using various modes of transportation?
% Basis: if we can travel from X to Y directly by car, train or plane then we can travel from X to Y
travel(X, Y) :- byCar(X, Y); byTrain(X, Y); byPlane(X, Y).
% Inductive Step 1: If we can travel directly by car from X to some Z such that we can also travel from Z to Y then we can travel from X to Y
travel(X, Y) :- byCar(X, Z), travel(Z, Y).
% Inductive Steps 2/3: Similar rules hold for trains and planes
travel(X, Y) :- byTrain(X, Z), travel(Z, Y).
travel(X, Y) :- byPlane(X, Z), travel(Z, Y).