Re: [Caml-list] List of structurally typed objects

[Damien Guichard <alphablock@orange.fr>]

Hi Richard.

Is this the code you want ?

type 'a obj_list = 'a list
constraint 'a = < hello : string; .. > as 'a

hope it helps,

Damien Guichard (SpiceGuid)

Richard W.M. Jones a écrit :
> 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.
>