R 创建包含相同类型插槽的S4类

作为实践，我正在使用S4类在R中创建LinkedList实现

我有以下课程：

setClass("node",

  slots = list(
    value = "numeric",
    next_node = "node"
  ),

  prototype = list(
    value = NA_real_,
    next_node = NA
  ) 
)

但是，我收到以下错误消息：

Error in makePrototypeFromClassDef(properties, ClassDef, immediate, where) : 
  in making the prototype for class “node” elements of the prototype failed to match the corresponding slot class: next_node (class "node" )
In addition: Warning message:
undefined slot classes in definition of "node": next_node(class "node")

您可以使用：

setClass("node",

  slots = list(
    value = "numeric",
    next_node = "nullOrNode"
  ),

  prototype = list(
    value = NA_real_,
    next_node = NULL
  ) 
)

setClassUnion("nullOrNode", c("NULL", "node"))