[Caml-list] List of structurally typed objects

["Richard W.M. Jones" <rich@annexia.org>]

Is it possible to construct a list of structurally typed ("duck
typed") objects?

The code below seems as if superficially it should work, at least as
far as I understand the OCaml memory model and how objects work
internally.  However the implementation doesn't match the interface.

------ list_fns.mli ----------------------------
type 'a obj = < hello : string; .. > as 'a
val register_obj : string -> 'a obj -> unit
val get_obj : string -> 'a obj
------------------------------------------------

------ list_fns.ml -----------------------------
type 'a obj = < hello : string; .. > as 'a
let objs = ref []
let register_obj name obj = objs := (name, obj) :: !objs
let get_obj name = List.assoc name !objs
------------------------------------------------

$ ocamlc -c list_fns.mli
$ ocamlc -c list_fns.ml
File "list_fns.ml", line 1:
Error: The implementation list_fns.ml
       does not match the interface list_fns.cmi:
       Values do not match:
         val register_obj : string -> '_a -> unit
       is not included in
         val register_obj : string -> < hello : string; .. > obj -> unit
       File "list_fns.ml", line 3, characters 4-16: Actual declaration

I'm guessing that "constraint" should be used here, but how?

Rich.