Comment author: @alainfrisch

Side note: the trunk version syntactically allows the suggested form, which is then rejected by the type-checker.

Comment author: @yallop

This will be supported naturally if exceptions are subsumed by open types.

Comment author: @yallop

This has been resolved by PR5584. Here's an example use, which now works in trunk:

module type INT =
sig
type t
val to_string : t -> string
val div : t -> t -> t
val zero : t
end

type 'i intmod = (module INT with type t = 'i)

exception Division_by_zero : 'a intmod * 'a -> exn

let div : type i. i intmod -> i -> i -> i =
fun ((module I) as i) n d ->
if d = I.zero then raise (Division_by_zero (i, n))
else I.div n d

let f (type i) ((module I : INT with type t = i) as i) x y =
match div i x y with
z ->
Printf.printf "%s / %s => %s\n"
(I.to_string x) (I.to_string y) (I.to_string z)
| exception Division_by_zero ((module I), n) ->
Printf.printf "%s / 0 => ERROR\n" (I.to_string n)

(* prints "12 / 4 => 3" *)
let _ = f (module Int32) 12l 4l

(* prints "12 / 0 => ERROR" *)
let _ = f (module Int64) 12L 0L