caml-list - the Caml user's mailing list
 help / Atom feed
From: Chet Murthy <murthy.chet@gmail.com>
To: Josh Berdine <josh@berdine.net>, caml-list <caml-list@inria.fr>
Cc: "Richard W.M. Jones" <rich@annexia.org>
Subject: Re: [Caml-list] Syntax puzzle / suggestion
Date: Sat, 10 Aug 2019 13:34:21 -0700
Message-ID: <CA++P_gcOycoK0JkAw+sWmJsM4=rd8wNZ1JYon-nJtbRD=Cbycw@mail.gmail.com> (raw)
In-Reply-To: <BD3A01EE-3256-4C45-A217-FF2B6D659FBA@berdine.net>

[-- Attachment #1: Type: text/plain, Size: 1821 bytes --]

Slight fixups:

let all_items = ref ([] : string list)
let add_item item = (all_items := item :: !all_items ; item)

let item1 = add_item "hello"
let item2 = add_item "world"

let all_items = List.rev !all_items


On Fri, Aug 9, 2019 at 7:53 AM Josh Berdine <josh@berdine.net> wrote:

> What about good old-fashioned state:
>
> let all_items = ref []
> let add_item item = all_items := (item :: !all_items, item)
>
> let item1 = add_item "hello"
> let item2 = add_item "world"
>
> let all_items = List.rev !all_items
>
>
> On 9 Aug 2019, at 15:27, Richard W.M. Jones wrote:
>
> > Let's imagine you have a list of named things, but you also want to
> > collect them up into a single list.  For example:
> >
> >   let item1 = "hello"
> >   let item2 = "world"
> >   let all_items = [ item1; item2 ]
> >
> >   let () =
> >     printf "item1 = %s\n" item1;
> >     printf "all_items = %s\n" (String.concat ", " all_items)
> >
> > This is fine, but there's a danger that a programmer could add item3
> > but forget to add it to the "all_items" list.  (In the real world
> > problem to which this applies, my items are complex and lengthy
> > structures, and the "all-things-list" is well off the bottom of the
> > page when you're viewing the top item).
> >
> > My idea to fix this was to write:
> >
> >   let all_items = [
> >     ("hello" as item1);
> >     ("world" as item2);
> >   ]
> >
> > Actually I was slightly surprised to find this doesn't compile.  I
> > guess because "as" can only be used like this in patterns, not
> > expressions.  Also the scoping is wrong because the "as item1" if it
> > worked probably wouldn't apply outside the list.
> >
> > I haven't worked out if it's possible to write this in ordinary OCaml,
> > although I suppose ppx could solve it.  Any ideas if this is possible?
> >
> > Rich.
>

[-- Attachment #2: Type: text/html, Size: 2605 bytes --]

  parent reply index

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-09 14:27 Richard W.M. Jones
2019-08-09 14:39 ` Gabriel Scherer
2019-08-09 14:45   ` Richard W.M. Jones
2019-08-09 14:48 ` Martin Riener
2019-08-10  5:19   ` rixed
2019-08-09 14:53 ` Josh Berdine
2019-08-10 15:18   ` Oliver Bandel
2019-08-10 20:34   ` Chet Murthy [this message]
2019-08-09 14:56 ` Frédéric Bour

Reply instructions:

You may reply publically 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='CA++P_gcOycoK0JkAw+sWmJsM4=rd8wNZ1JYon-nJtbRD=Cbycw@mail.gmail.com' \
    --to=murthy.chet@gmail.com \
    --cc=caml-list@inria.fr \
    --cc=josh@berdine.net \
    --cc=rich@annexia.org \
    /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

caml-list - the Caml user's mailing list

Archives are clonable: git clone --mirror https://inbox.ocaml.org/caml-list

AGPL code for this site: git clone https://public-inbox.org/ public-inbox