Oct 31, 2003
-------------

- More on PROLOG
	- Using append in interesting ways
		- member
		- nextto
		- sublist
		- triple
		- reverse
		- bubblesort (fix this, it is giving spurious
			     answers after finding the right one)
	- ancestor
		- order of clauses matter
		- left-recursion is bad, since Prolog uses DFS!
	- a halloween party with predicates
		- mutual recursion is bad,
		  doesn't cause stack overflow, but infinite loop
		  nevertheless

Files used in this session
<lists.pl>
/* function to append lists */
/* first two are the inputs, last is the
appended list */

append([],X,X).
append(X,[],X).
append([X|Y],Z,[X|M]) :- append(Y,Z,M).

/* function to find the last element of
a list */

last(X,List) :- append(_,[X],List).

/* function to check if X is a member
of list List */
member(X,List) :- append(_,[X|_],List).

/* function to check if X and Y are
next to each other in a list L */
nextto(X,Y,L) :- append(_,[X,Y|_],L).


/* function to check if X is a sublist
of Y */

sublist(X,Y) :- append(_,X,W), append(W,_,Y).

/* function to triple a list */
/* given [3] produce [3,3,3] as answer */
/* X when tripled gives list Y */

triple(X,XXX) :- append(X,X,XX),
                 append(XX,X,XXX).

/* function to reverse a list */
/* give X as input, Y is the reversed X */

reverse([],[]).
reverse([A|B],Y) :- reverse(B,Z),
                    append(Z,[A],Y).

/* function to do bubblesort */
/* X when bubblesorted gives Y */

bubblesort(X,Y) :- append(M,[A,B|N],X),
                   A > B,
                   append(M,[B,A|N],S),
                   bubblesort(S,Y).
bubblesort(L,L).
/* fix this above code yourself, so as to
not give more answers after the correct
one */

<family.pl>
/* in ancestor(X,Y), Y is meant to be the
ancestor of X */

/* order of predicate definitions matter */
/* beware of left recursion, as written below */
/* because Prolog uses depth-first search */

ancestor(X,X).
ancestor(X,Y) :- ancestor(Z,Y), parent(X,Z).
parent(amy,mark).

<halloween.pl>
trick(eggs).
trick(toiletpaper).
trick(mailboxbaseball).
trick(pumpkinsmashing).

treat(kitkat).
treat(hersheys).
treat(snickers).

practice(oct31, eggs).
practice(oct31, toiletpaper).

meal(oct31, hersheys).

niceday(X) :- classletsoffearly(X).
classletsoffearly(X) :- niceday(X).