On 10/18/2016 12:21 PM, Goswin von Brederlow wrote:
> You can pass along the outermost to_string to all to_string
> functions so they can restart from the outside when they recurse.

Yep :) For the record, I got it working like this:

In M0:

  let assert_false _ = assert false
  type f = { f : 'a. 'a t -> string }

  let rec to_string
      : type a b. ?f:f -> a t -> string
      = fun ?(f = { f = assert_false }) e -> match e with
      | I i -> "I:" ^ string_of_int i
      | S s -> "S:" ^ s
      | P (x, y) -> to_string ~f x ^ to_string ~f y
      | e -> f.f e

In M1:

    let rec to_string' : type a. a t -> string = function
      | T t -> to_string ~f:{ f = to_string' } t
      | x -> to_string ~f:{ f = to_string' } x

    let to_string = to_string'

Cheers,
Andre