From: "Stéphane Glondu" <steph@glondu.net>
To: Dario Teixeira <darioteixeira@yahoo.com>
Cc: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] Structural subtyping problem
Date: Sun, 28 Mar 2010 19:32:37 +0200 [thread overview]
Message-ID: <4BAF92B5.1060502@glondu.net> (raw)
In-Reply-To: <61102.44294.qm@web111503.mail.gq1.yahoo.com>
Dario Teixeira a écrit :
> 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 >".
As said elsewhere, this is because you are trying to do polymorphic
recursion. Standard tricks work here:
------------------------------------------------------------------------
type steps = {
step1 : 'a. ?story:(<title : string; ..> as 'a) -> unit -> bool;
step2 : string -> bool;
step3 : 'a. story:(<title : string; count : int; ..> as 'a) -> bool
}
let rec steps = {
step1 =
begin fun ?story () ->
match story with
| Some s -> steps.step2 s#title
| None -> steps.step2 "title1"
end;
step2 =
begin fun title ->
let story = object
method title = title
method count = 0
end in
steps.step3 story
end;
step3 =
begin fun ~story ->
match story#count with
| 0 ->
steps.step1 ~story ()
| 1 ->
let story = object
method title = "title2"
end in
steps.step1 ~story ()
| _ ->
true
end
}
let step1 = steps.step1
let step2 = steps.step2
let step3 = steps.step3
------------------------------------------------------------------------
With OCaml 3.12, I guess this should be feasible without using the
intermediate "steps" record.
Cheers,
--
Stéphane
next prev parent reply other threads:[~2010-03-28 17:32 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-03-28 16:38 Dario Teixeira
2010-03-28 17:03 ` [Caml-list] " Vincent Aravantinos
2010-03-28 17:07 ` Andreas Rossberg
2010-03-28 17:32 ` Stéphane Glondu [this message]
2010-03-28 19:20 ` Christophe TROESTLER
2010-03-28 20:17 ` Dario Teixeira
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=4BAF92B5.1060502@glondu.net \
--to=steph@glondu.net \
--cc=caml-list@yquem.inria.fr \
--cc=darioteixeira@yahoo.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).