Pretty print a tree

Question

Let\'s say I have a binary tree data structure defined as follows

type \'a tree =
    | Node of \'a tree * \'a * \'a tree
    | Nil

I have

Answer 1

Although it's not exactly the right output, I found an answer at http://www.christiankissig.de/cms/files/ocaml99/problem67.ml :

(* A string representation of binary trees

Somebody represents binary trees as strings of the following type (see example opposite):

a(b(d,e),c(,f(g,)))

a) Write a Prolog predicate which generates this string representation, if the tree 
is given as usual (as nil or t(X,L,R) term). Then write a predicate which does this 
inverse; i.e. given the string representation, construct the tree in the usual form. 
Finally, combine the two predicates in a single predicate tree_string/2 which can be 
used in both directions.

b) Write the same predicate tree_string/2 using difference lists and a single 
predicate tree_dlist/2 which does the conversion between a tree and a difference 
list in both directions.

For simplicity, suppose the information in the nodes is a single letter and there are 
no spaces in the string. 
*)

type bin_tree = 
    Leaf of string
|   Node of string * bin_tree * bin_tree
;;

let rec tree_to_string t =
    match t with
            Leaf s -> s
    |       Node (s,tl,tr) -> 
                    String.concat "" 
                            [s;"(";tree_to_string tl;",";tree_to_string tr;")"]
;;