为什么我不能在数字上定义方法？

Question

在Ruby中，我可以在对象上定义方法：

x = "asht"

def x.roman
  "works!"
end
x.roman # => "works"

"asht".roman # => NoMethodError: undefined method `roman' for "asht":String

但是，这不适用于数字

def 0.roman
  "doesn't work :'("
end # => SyntaxError: (irb):5: syntax error, unexpected tINTEGER

x = 0
def x.roman
  "doesn't work :'("
end # => TypeError: can't define singleton

为什么？