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Date: Sun, 28 Mar 2010 09:38:07 -0700 (PDT)
From: Dario Teixeira <darioteixeira@yahoo.com>
Subject: Structural subtyping problem
To: caml-list@yquem.inria.fr
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X-Spam: no; 0.00; subtyping:01 subtyping:01 ocaml's:01 intuitively:01 compiler:01 annotations:01 coercions:01 coercions:01 rec:01 compile:01 int:01 emulate:01 width:97 parameter:02 parameter:02 

Hi,=0A=0AI'm using the structural subtyping aspects of Ocaml's object syste=
m to emulate width=0Asubtyping.  I've come across a problem which does not =
type-check, though intuitively=0Ait seems correct.  I reckon that the compi=
ler may need some help in the form of type=0Aannotations and/or coercions, =
though their exact shape elludes me.=0A=0AA simple nonsensical example that=
 illustrates the problem is listed below; the=0Atype-checking error occurs =
in function "step1", where the optional parameter "story"=0Ais used as an o=
bject of type "< title:string; .. >".  In function "step3", this=0Aparamete=
r "story" is actually instantiated with objects of type "< title:string >"=
=0Aand "< title:string; count:int >".=0A=0AAnyway, am I correct in assuming=
 this should be feasible?  And if so, what coercions=0Aare required to make=
 this compile?=0A=0AThanks in advance!=0ABest regards,=0ADario Teixeira=0A=
=0A=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=0A=0Alet rec step1 ?story () =3D match story with=0A=09| Some s -=
> step2 s#title=0A=09| None   -> step2 "title1"=0A=0A=0Aand step2 title =3D=
=0A=09let story =3D=0A=09=09object=0A=09=09=09method title =3D title=0A=09=
=09=09method count =3D 0=0A=09=09end=0A=09in step3 ~story=0A=0A=0Aand step3=
 ~story =3D match story#count with=0A=09| 0 ->=0A=09=09step1 ~story ()=0A=
=09| 1 ->=0A=09=09let story =3D=0A=09=09=09object=0A=09=09=09=09method titl=
e =3D "title2"=0A=09=09=09end=0A=09=09in step1 ~story ()=0A=09| _ ->=0A=09=
=09true=0A=0A=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=0A=0A=0A=0A=0A