?- use_module(library(random)).


% this predicate loops as follows: It asks the user for a cmd. If it is
% listing, a listing is done. If random, then a random number is
% written. If it is end, then the loop quits. Anything else, and an
% error message is written.

% This version uses a failure-driven loop to do iteration.
% Looping is started with repeat, and continues until quit is entered.
% Cuts are used to prevent backtracking, which also reduces memory usage
% when recursing, as there is no need to retain the tree for backtrack.

cmdLooper :-
	repeat,
	write('enter command (listing, random, or quit): '),
	read(Term),
	do_cmd(Term),
	Term = quit.

do_cmd(quit) :-
	write('Quitting.'), nl.
do_cmd(listing) :-
	!,
	write('Listing...'),
	nl,
	listing.
do_cmd(random) :-
	!,
	write('Random number: '),
	random(X),
	write(X),
	nl.
do_cmd(X) :-
	write('No siree Bob! Bad command: '), 
	write(X),
	write('.'),
	nl.