OCaml not accepting type constructor for binary tree

Question

I am trying to construct something like this

type ('a, 'b) btree = Empty | Node of ('a, 'b) * ('a, 'b) btree * ('a, 'b) btree

but it says syntax error around where the first asterisk is.

Answer 1

The type of a pair looks like 'a * 'b so you want this:

type ('a, 'b) btree = 
  Empty |
  Node of 'a * 'b * ('a, 'b) btree * ('a, 'b) btree

Or possibly this:

type ('a, 'b) btree = 
  Empty |
  Node of ('a * 'b) * ('a, 'b) btree * ('a, 'b) btree