class Module
    def before_method method, &block
        alias_method "__old__#{method.to_s}".to_sym, method unless respond_to? "__old__#{method.to_s}".to_sym
        define_method method do |*args|
            block.call(*args)
            send "__old__#{method.to_s}".to_sym, *args
        end
    end
end

class Test
    def greet who
        puts "hello #{who}"
    end
end

Test.before_method :greet do |w|
    puts "Calling a greeting for #{w}"
end

b = Test.new
b.greet "world"