programming-lua/exercises/ch01/ex01/main.lua

-- Negative n resulted in a stackoverflow (infinite recursion)
function fact(n)
    if n == 0 then
        return 1
    elseif n < 0 then
        return nil
    else
        return n * fact(n - 1)
    end
end

print("enter a number:")
a = io.read("*n")
print(fact(a))