Structural subtyping problem

[Dario Teixeira <darioteixeira@yahoo.com>]

Hi,

I'm using the structural subtyping aspects of Ocaml's object system to emulate width
subtyping.  I've come across a problem which does not type-check, though intuitively
it seems correct.  I reckon that the compiler may need some help in the form of type
annotations and/or coercions, though their exact shape elludes me.

A simple nonsensical example that illustrates the problem is listed below; the
type-checking error occurs in function "step1", where the optional parameter "story"
is used as an object of type "< title:string; .. >".  In function "step3", this
parameter "story" is actually instantiated with objects of type "< title:string >"
and "< title:string; count:int >".

Anyway, am I correct in assuming this should be feasible?  And if so, what coercions
are required to make this compile?

Thanks in advance!
Best regards,
Dario Teixeira

=============================================================================

let rec step1 ?story () = match story with
	| Some s -> step2 s#title
	| None   -> step2 "title1"


and step2 title =
	let story =
		object
			method title = title
			method count = 0
		end
	in step3 ~story


and step3 ~story = match story#count with
	| 0 ->
		step1 ~story ()
	| 1 ->
		let story =
			object
				method title = "title2"
			end
		in step1 ~story ()
	| _ ->
		true

=============================================================================