The ruby comparison is unfair:

    o = Object.new
    def o.my_method(x)
      self.y + x
    end
    def o.y
      10
    end
    o.my_method(5) == 15 # true
    method_as_fun = o.my_method
    # Wrong number of arguments, 0 for 1

The last line is actually doing: method_as_fun = o.my_method()

Which should make the error message obvious. Ruby makes this syntax optional for readability purposes, which becomes obvious once you start going through real world ruby code.