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%"Spider" -- A Sample Adventure Game in Prolog
% David Matuszek, Villanova University
% http://www.csc.vill.edu/~dmatusze/resources/prolog/spider.html

% This defines my current location

i_am_at(meadow).

% These facts describe how the rooms are connected.

path(spider, d, cave).
path(cave, u, spider).

path(cave, w, cave_entrance).
path(cave_entrance, e, cave).

path(cave_entrance, s, meadow).
path(meadow, n, cave_entrance) :- at(flashlight, in_hand).
path(meadow, n, cave_entrance) :-
        print('Go into that dark cave without a light?  Are you crazy?'), nl,
        fail.

path(meadow, s, building).
path(building, n, meadow).

path(building, w, cage).
path(cage, e, building).

path(closet, w, building).
path(building, e, closet) :- at(key, in_hand).
path(building, e, closet) :-
        print('The door appears to be locked.'), nl,
        fail.

% These facts tell where the various objects in the game are located.

at(ruby, spider).
at(key, cave_entrance).
at(flashlight, building).
at(sword, closet).

% This fact specifies that the spider is alive.

alive(spider).

% These rules describe how to pick up an object.

take(X) :-
        at(X, in_hand),
        print('You are already holding it!'),
        nl.

take(X) :-
        i_am_at(Place),
        at(X, Place),
        retract(at(X, Place)),
        assert(at(X, in_hand)),
        print('OK.'),
        nl.

take(_) :-
        print('I do not see it here.'),
        nl.