% The following code is derived from the Mercury Tutorial by Ralph Becket.
% http://www.mercury.csse.unimelb.edu.au/information/papers/book.pdf
:- module fib.

:- interface.
:- import_module io.
:- pred main(io::di, io::uo) is det.

:- implementation.
:- import_module int.

:- pred fib(int::in, int::out) is det.
fib(N, X) :-
    ( if N =< 2
          then X = 1
          else fib(N - 1, A), fib(N - 2, B), X = A + B ).

:- func fib(int) = int is det.
fib(N) = X :- fib(N, X).

main(!IO) :-
    fib(40, X),
    write_string("fib(40, ", !IO),
    write_int(X, !IO),
    write_string(")\n", !IO),

    write_string("fib(40) = ", !IO),
    write_int(fib(40), !IO),
    write_string("\n", !IO).