From: "Chris King" <colanderman@gmail.com>
To: "David Allsopp" <dra-news@metastack.com>
Cc: "OCaml List" <caml-list@yquem.inria.fr>
Subject: Re: Polymorphic variants question
Date: Fri, 1 Sep 2006 14:33:53 -0400 [thread overview]
Message-ID: <875c7e070609011133udd506c7v223d8ece952b95ae@mail.gmail.com> (raw)
In-Reply-To: <012901c6cdec$64edf490$6a7ba8c0@treble>
On 9/1/06, David Allsopp <dra-news@metastack.com> wrote:
> Now, if I try to constrain it to what I'm after with
>
> let (f : [`A | `C] -> bool * [`A | `B | `C]) = fun x -> ...
>
> then I get a type error unless I change
> (false, x)
> to
> (false, id x)
> with
> let id = function `A -> `A | `C -> `C
>
> Is there a better way of writing this?
Yes, you can use the coercion operator to tell the compiler to expand the type of x to include `B:
# let f (x: [`A | `C]) = if x = `A then (true, `B) else (false, (x :> [`A | `B | `C]))
val f : [ `A | `C ] -> bool * [ `A | `B | `C ] = <fun>
Perhaps someone else can clarify how this accomplishes the same thing as your id function, as my understanding of type theory is somewhat rudimentary.
- Chris King
next prev parent reply other threads:[~2006-09-01 18:33 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-09-01 17:31 David Allsopp
2006-09-01 18:33 ` Chris King [this message]
2006-09-01 19:00 ` [Caml-list] " skaller
2006-09-01 19:57 ` David Allsopp
2006-09-01 20:40 ` Jacques Carette
2006-09-01 18:40 ` [Caml-list] " Olivier Andrieu
2006-09-01 19:26 ` Jon Harrop
2006-09-01 19:29 ` skaller
2006-09-01 20:49 ` Andres Varon
2006-09-02 11:16 ` Jacques Garrigue
2006-09-03 1:22 ` Andres Varon
2006-09-03 9:08 ` Jacques Garrigue
2006-09-03 15:00 ` Andres Varon
2006-09-03 23:18 ` Jacques Garrigue
2006-09-03 0:48 ` [Caml-list] Polymorphic variants problem skaller
2006-09-03 1:12 ` Andres Varon
-- strict thread matches above, loose matches on Subject: below --
2000-04-26 0:25 Polymorphic variants question Brian Rogoff
2000-04-26 9:34 ` Jacques Garrigue
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=875c7e070609011133udd506c7v223d8ece952b95ae@mail.gmail.com \
--to=colanderman@gmail.com \
--cc=caml-list@yquem.inria.fr \
--cc=dra-news@metastack.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).