Turbo Prolog

Program 34 uses standard graphics predicates to construct a "doodle pad." A border is
drawn around the screen, and you can draw by pressing the CD (for Up), []J (for
Down), El (for Left), and El (for Right) keys. Try the program out before examining
the code. Keys other than ([), []J, !Il and [[) are ignored, except that pressing 0
exits the program.
/1 Program 3t; 1/
predicates
move(char,integer,integer,integer)
start
changestate(integer,integer)
goal
start
clauses
start:graphics(l,l,t;),
line(1000,1000,1000,31000,2),
line(1000,31000,31000,31000,2),
line(31000,31000,31000,1000,2),
line(31000,1000,1000,1000,2),
changestate(15000,15000).
changestate(X,Y):readchar(Z),move(Z,X,Y,Xl,Yl),changestate(Xl,Yl).
move('r',X,31000,X,31000):- !.
move('r',X,Yold,X,Ynew):- !,Ynew=Yold+l00,dot(X,Yold,3).
move('l',X,1000,X,1000):- !.
move('l',X,Yold,X,Ynew):- !,Ynew=Yold-l00,dot(X,Yold,3).
move('v',1000,Y,1000,Y):- !.
move('v';Xold,Y,Xnew,Y):- !,Xnew=Xold-l00,dot(Xold,y,3).
move('d',31000,Y,31000,Y):- !.
move('d',Xold,Y,Xnew,Y):- !,Xnew=Xold+l00,dot(Xold,Y,3).
move('I',_,_,_,_):- !,exit.
move(_,X,y,X,y).
Turtle Graphics Commands
Standard predicates that produce effects similar to Program 34 are built into Turbo
Prolog-the Turtle Graphics commands.
When you enter graphics mode the screen is cleared, and a turtle appears in the
middle of the screen facing the top of the screen vertically and with a "pen" attached to
its tail. As the turtle is directed to move by various standard predicates, the "pen"
leaves a trail on the screen.
The effect of these predicates depends on
• The position of the turtle
• The direction of the turtle
• Whether the "pen" is drawing (activated) or not
• The color of the pen
The standard predicate pendown activates the pen and penup deactivates it. Immediately
after a call to graphics, the pen is activated. The color of the trail left by the pen
Tutorial VI: Graphics and Sound 93