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.