how to define two types referring to each other in OCaml?

Question

The code below will report Syntax error message:

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

How to define two types referring to each other?

This is a commonly asked question. Here's a link to a similar one: http://stackoverflow.com/questions/3026123/ocaml-forward-declaration — Jeffrey Scofield, Mar 28 '12 at 01:18

score 8 · Accepted Answer · answered Mar 28 '12 at 01:16

8

The second type is not syntactically correct:

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

answered Mar 28 '12 at 01:16

Rafe Kettler

2

(The key is that you need to use `and` to define mutually recursive types.) – Jeffrey Scofield Mar 28 '12 at 01:21

1 Answers1