Re: [Caml-list] Syntax puzzle / suggestion

[Chet Murthy <murthy.chet@gmail.com>]

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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.
>

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