New Collections interface - Sized

[Stephen Colebourne]

On Fri, 23 Apr 2021 at 17:08, Brian Goetz <brian.goetz at oracle.com> wrote:
> This has come up before.  For example, during an early iteration of the Stream design, before parallelism entered the picture.  The first scrawled-on-a-napkin prototype for streams was based on Iterator, and it took about a minute to realize that we could do a much better job if we had a slightly broader interface to work with, essentially Iterator+Sized.
>
> When you pull on this string, you end up with a lot of new interfaces, such as SizedIterator, SizedIterable, etc, in part because ... we have no intersection types.  Having lots of fine-grained interfaces for "has X" and "has Y" is nice from a "bucket of lego bricks" library-design perspective, but when the user goes to express "I need an aggregate that has sizes, iterators, and encounter order", you end up with code like:
>
>     <T, X extends Iterable<T>&Sized> void foo(X x) { ... }
>
> and then you run into the wall of "but I can only use intersection types in these places in the language."  The idiom of having fine-grained mix-in-ish interfaces really wants a language with intersection types.
>
> Additionally, I am having a hard time imagining how Sized would be usable by a client; no method will *take* a Sized (it's just not broad enough), and I can't immediately imagine what would even *return* a Sized.  If the type is not suitable for use by clients, then it serves only to organize the library itself, and that's a weaker motivation.
>
> Is there a compelling example of where this would be used by clients?

Here are some examples:
https://stackoverflow.com/questions/10988634/java-global-isempty-method
https://github.com/OpenGamma/Strata/blob/main/modules/collect/src/main/java/com/opengamma/strata/collect/ArgChecker.java#L404

ie. the ability to spot an "empty" object, or ensure input is
non-empty. Use cases are generally in low-level/framework code like
this, where the actual input data type is not known.

Stephen