I always found

  let [item1; item2] as all_items = ["hello"; "world" ]

readable but the compiler does does not infer that this match always works and
emits:

Warning 8: this pattern-matching is not exhaustive.
Here is an example of a case that is not matched:
(_::_::_::_|_::[]|[])

I've never touched the compiler so I don't have an idea how much effort it would
be to make the inference.

cheers,
Martin



On 8/9/19 4:27 PM, 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.
>