Ciao

:- module(hanoi, [hanoi/0, hanoi/1], [classic, assertions]).

:- comment(title, "The classical ``Towers of Hanoi'' puzzle").

:- comment(author, "The Ciao/Prolog documentation system processor").

:- comment(summary, "This program writes to the standard output a sequence of
   moves to solve the ``Towers of Hanoi'' puzzle for a given number of
   disks.").

:- comment(module, "This is a very simple program which is self-documented
   and also includes some checking assertions. It is included as a demo of
   the Ciao assertion language and preprocessor, and of the Ciao
   documentation generator.").

:- pred hanoi/0.

hanoi :-
        hanoi(5).

:- pred hanoi(N) : int(N) => true.

hanoi(N) :-
        move(N, left, center, right).

:- pred move(N, A, B, C) : (int(N), pole(A), pole(B), pole(C)) => true
   # "Move @var{N} disks from pole @var{A} to pole @var{B} using
     @var{C} as auxiliary.".

move(0, _, _, _) :- !.
move(N, A, B, C) :-
        N > 0,
        M is N - 1,
        move(M, A, C, B),
        inform_user(A, B),
        move(M, C, B, A).

:- pred inform_user(A, B) : (pole(A), pole(B)) => true
   # "Inform the user of a move from pole @var{A} to pole @var{B}.".

inform_user(A, B) :-
        display('Move a disk from '),
        display(A),
        display(' to '),
        display(B),
        nl.

:- type pole ---> left ; center ; right.