Nov 3, 2003
-------------

- Imparting more control knowledge in Prolog
	- cut operator (!)

- Many uses of cut
	- prevent consideration of alternate solutions
	- "freezing" parts of solutions
	- by itself evaluates to true
	- prevent multiple solutions from being produced

- Files for today:
<skinner.pl>
/* squarelist takes a list of integers
and squares every entry */

/* ! is called "cut". It is the goto of
Prolog. Use it with great precaution. */

/* cut prevents consideration of alternate
solutions by freezing parts of current
solution */

/* cut here is doing
        - fix rule 2 when you know A is
          an integer
        - skip all subsequent rules
          (in this case, rule 3)
*/

squarelist([],[]).
squarelist([A|B],[C|D]) :- integer(A), !,
                           C is A*A,
                           squarelist(B,D).
squarelist([A|B],[A|D]) :- squarelist(B,D).
<green.pl>
/* cut by itself always evalutes to true
*/

drink(beer).
drink(gin). :- !
drink(juice).

/* try the above code with the following queries:
drink(X).
drink(X), drink(Y).
drink(X), ?, drink(Y) */

/* cut prevents multiple solutions from
being produced. */

member(X,[X|_]) :- !.
member(X,[_|Y]) :- member(X,Y).

/* max - first two are input arguments,
last is the result of max */

/* max(X,Y,X) :- X>=Y.
max(X,Y,Y) :- X<Y. */

/* don't try to simulate if-then-else
loop in Prolog. Make each clause stand
independently by itself. */

/* max(X,Y,X) :- X>=Y, !.
max(X,Y,Y). */

/*
        if x>=y then return(x)
        else return (y)
*/

/* the nice way to write the max predicate is: */
max(X,Y,X) :- X>=Y, !.
max(X,Y,Y) :- X<Y.

/* "if A then B else C"
       - in C-like language

In Prolog-like notation, this will
look like:

B :- A, !.
C :- not(A). */