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From: "Richard W.M. Jones" <>
Subject: [Caml-list] Syntax puzzle / suggestion
Date: Fri, 9 Aug 2019 15:27:20 +0100
Message-ID: <> (raw)

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?


             reply index

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-09 14:27 Richard W.M. Jones [this message]
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
2019-08-09 14:56 ` Frédéric Bour

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