如何在OCaml中定义两个相互引用的类型？

Question

下面的代码将报告语法错误消息:

type 'a edge = 
  |Empty 
  |End of 'a * 'a vertex * 'a vertex and
type 'a vertex = 
  |Empty
  |Vertex of 'a * 'a edge list;;

如何定义两种相互引用的类型？

Answer 1

第二个type在语法上不正确:

type 'a edge = 
  |Empty 
  |End of 'a * 'a vertex * 'a vertex
and 'a vertex = 
  |Empty
  |Vertex of 'a * 'a edge list;;