Discontiguous predicate warning from GNU Prolog

Question

For a lark I have started learning Prolog via http://www.learnprolognow.org/. My excuse is that I have only just started.

I am unsure of how the following (simple?) exercise should be written to work as expected. http://cs.union.edu/~striegnk/learn-prolog-now/html/node13.html#sec.l1.exercises

I can see that Gnu Prolog 1.4 ( http://www.gprolog.org/ ) is not happy that the 2nd predicate for wizard\1 is not next to the first one. I can re-order the program to make it work but then I end up with code which to my novice Prolog eye is less understandable than the one presented below.

Am I missing something obvious?

wiz.pl

wizard(ron).
hasWand(harry).
quidditchPlayer(harry).

wizard(X) :- hasBroom(X),hasWand(X).
hasBroom(X) :- quidditchPlayer(X).

consult wiz.pl

| ?- [wiz].
compiling D:/wiz.pl for byte code...
D:/wiz.pl:5: warning: discontiguous predicate wizard/1 - clause ignored
D:/wiz.pl compiled, 5 lines read - 632 bytes written, 24 ms

Answer 1

Most Prologs want the clauses for any particular predicate to be listed together, unless you do some magic. I'm surprised Striegnitz and Bos ignore this. Change the program to

% all clauses for wizard/1
wizard(ron).
wizard(X) :- hasBroom(X),hasWand(X).

hasWand(harry).
quidditchPlayer(harry).
hasBroom(X) :- quidditchPlayer(X).

Note that I've kept the clauses for wizard/1 in the same order as in the original program. For simple knowledge bases such as this one, order doesn't matter much, but when implementing non-deterministic algorithms, clause order may determine the order in which solutions are generated.