Hi Richard,

Note that you can still write < hello: string; .. > in the signature of
register_obj and do the cast before adding your object to the list:

  let register_obj name obj = objs := (name, (obj :> < hello: string >)) ::
!objs

This way you do not need to downcast at each call-site of register_obj.

Cheers,
Nicolas



On Wed, Feb 22, 2017 at 6:36 PM, Richard W.M. Jones <rich@annexia.org>
wrote:

> On Wed, Feb 22, 2017 at 12:09:37PM -0500, Gabriel Scherer wrote:
> > 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.
>
> Ah, I see.
>
> Modifying the code to use the monomorphic type works, but I have to
> downcast the objects to the right type.  For reference, the working
> code is below.
>
> Thanks,
>
> Rich.
>
> list_fns.mli --------------------------------------------------------
> type obj = < hello : string >
> val register_obj : string -> obj -> unit
> val get_obj : string -> obj
>
> list_fns.ml ---------------------------------------------------------
> type obj = < hello : string >
> let objs = ref []
> let register_obj name obj = objs := (name, obj) :: !objs
> let get_obj name = List.assoc name !objs
>
> test.ml -------------------------------------------------------------
> class foo = object
>   method hello = "hello from foo"
>   method goodbye () = print_endline "goodbye"
> end
>
> class bar = object
>   method hello = "hello from bar"
> end
>
> let () =
>   let o1 = new foo in
>   let o2 = new bar in
>   List_fns.register_obj "o1" (o1 :> List_fns.obj);
>   List_fns.register_obj "o2" (o2 :> List_fns.obj);
>   print_endline ("calling o1: " ^ (List_fns.get_obj "o1")#hello);
>   print_endline ("calling o2: " ^ (List_fns.get_obj "o2")#hello)
> --------------------------------------------------------------------
>
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