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

[Gabriel Scherer <gabriel.scherer@gmail.com>]

The signature you demand is actually incorrect, because the return
type of get_obj is to permissive: it claims that it can return *any*
('a obj), so you could register an object with just the hello method
and get back an object with more methods.

On the other hand, if you use the monomorphic < hello : string > type
in the interface, then it is type-correct and the implementation you
provide is accepted. It is explicit in the fact that you can't recover
the extra methods of the object once they have been stored in this
lowest-common-denominator container.

On Wed, Feb 22, 2017 at 12:01 PM, Richard W.M. Jones <rich@annexia.org> wrote:
> 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.
>
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