Custom Struct方法无法设置struct成员

一个例子将解释这个问题:

Val = Struct.new(:value) do
  def inc 
    p value
    value = value + 1 
  end
end

v = Val.new(1)
v.inc

输出将是:

1
undefined method `+' for nil:NilClass (NoMethodError)

为什么在value明显不为零时会出现此错误？有没有办法让这项工作？

Val = Struct.new(:value) do
  def inc 
    p value # here it still prints 1

    # but here you REDEFINED what value is. It is now a local variable!
    # Also its initial value is nil, hence the error you're getting.
    value = value + 1 

    # should have used this instead, to reference the method
    self.value = value + 1
  end
end