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/* "Sleepy" -- a sample adventure game, by David Matuszek. */

% http://www.csc.vill.edu/~dmatusze/resources/prolog/sleepy.html
/* In standard Prolog, all predicates are "dynamic": they
   can be changed during execution. SWI-Prolog requires such
   predicates to be specially marked. */

% :- dynamic at/2, i_am_at/1, i_am_holding/1, alive/1,
%           lit/1, visible_object/1.

/* This routine is purely for debugging purposes. */

%dump :- listing(at), listing(i_am_at), listing(i_am_holding),
%        listing(alive), listing(lit), listing(visible_object).

/* This defines my current location. */

i_am_at(bedroom).

i_am_holding(nothing).

/* These facts describe how the rooms are connected. */

path(bedroom, n, den) :- lit(bedroom).
path(bedroom, n, den) :-
    print('You trip over something in the dark.'), nl,
    !, fail.
path(den, s, bedroom).

path(bedroom, d, bed).
path(bed, u, bedroom).


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

at(flyswatter, den).
at(fly, bedroom).
at('light switch', den).
at('light switch', bedroom).


/* These facts specify some game-specific information. */

alive(fly).

lit(bedroom).
lit(den).

visible_object('light switch').

/* These rules describe how to pick up an object. */

take(fly) :-
    print('It is too fast for you!'), nl,
    !, fail.

take('light switch') :-
    take(switch).

take(switch) :-
    print('It is firmly embedded in the wall!'), nl,
    !, fail.

take(X) :-