Recursive travel

[EricGT]

@DaJa

When/how is transportation mode added to Path?

travel_01_helper(X,Y,Path0,[(Mode,Y)|Path0]) :-
    (
        (byCar(X,Y),Mode = car)
    ;
        (byTrain(X,Y),Mode = train)
    ;
        (byPlane(X,Y),Mode = plane)
    ),
    \+ member((_,Y),Path0).

The details can be seen by using trace/0 with protocol/1 as noted in the earlier post. Here I will just explain the details without showing the code or the trace.

The predicate travel_01_helper/4 is entered on the first line with the first three arguments bound, so [(Mode,Y)|Path0] is not fully bound but, Y and Path0 are bound because they were bound when the predicate was entered.

Next there are thee possibilities for the next statement because of the use of ;. When one of the three predicates, e.g. byCar(X,Y) succeeds/returns true, then the next predicate is called, e.g. Mode = car and via unification =/2, the unbound variable Mode is bound to the value.

In the example Mode = car, =/2 is used as an infix operator as opposed to a prefix operator.

?- Mode = car.
Mode = car.

?- =(Mode,car).
Mode = car.

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If you want to look at other somewhat simple Prolog examples see RosettaCode - Prolog

For other useful links see: Useful Prolog references