Abusing list notations and GADTs to keep track of length and ensure
exhaustive matching:

module M = struct type ('a, 'b) t = [] : (_, unit) t | (::) : 'a * ('a, 'b)
t -> ('a, unit * 'b) t end;;
let rec to_list : type a b. (a, b) M.t -> a list = function | M.[] -> [] |
M.(x :: xs) -> x :: to_list xs;;
let decompose x = (to_list x, x);;

# let all_items, M.[item1; item2] = decompose M.["hello"; "world"];;
val all_items : string list = ["hello"; "world"]
val item1 : string = "hello"
val item2 : string = "world"

On Fri, Aug 9, 2019 at 4:27 PM Richard W.M. Jones <rich@annexia.org> 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.
>