#!/usr/bin/env escript
%% -*- erlang -*-
%%! -smp enable -sname factorial -mnesia debug verbose
%%
%% From: https://www.erlang.org/docs/17/man/escript
main([String]) ->
    try
        N = list_to_integer(String),
        F = fac(N),
        io:format("factorial ~w = ~w\n", [N, F])
    catch
        _:_ ->
            usage()
    end;
main(_) ->
    usage().


usage() ->
    io:format("usage: factorial integer\n"),
    halt(1).


fac(0) -> 1;
fac(N) -> N * fac(N - 1).