> On Jan 10, 2020, at 11:03 AM, Brian Goetz <brian.go...@oracle.com> wrote:
>
> I don't want to dive into a design discussion on declared patterns, but I
> think we can resolve most of the nullability issues without doing so.
>
> Like methods, declared patterns are members. Like methods, they can be
> static or instance.
>
> If they are instance, the target is the receiver, and that surely won't work
> if the receiver is null. So by definition, instance patterns cannot match
> null. Let's set that aside.
>
> (Example: an instance pattern is a pattern on a class, like:
>
> switch (aString) {
> case palindrome(var firstHalf, boolean isOdd): ...
> }
>
> where palindrome is an instance pattern on String.)
>
> Declared patterns have a target type (may want a better name), which for an
> instance pattern is the receiver type, and for a static pattern is part of
> the declaration. (For example, `Optional.of(var x)` has a target type of
> `<T> Optional<T>`.)
>
> We won't even try to match the pattern unless the match target is of the
> target type for the pattern (it's not an Optional.of(anything) if its not an
> Optional to begin with.) So we have to do some sort of test first -- either
> instanceof (in which case static pattern will never match null, period), or a
> type pattern match (which would be the more principled choice, and would only
> admit a null through to the user-written code if the target type of the
> static pattern were total on the operand of the switch/instanceof.)
>
> So it's already a pretty narrow case where a null would be admitted into the
> user code at all. Now, the finishing blow: do we really want to ask the
> authors of the `Optional.of(var x)` and `Optional.empty()` patterns to have
> to check their target for null? That would be creating a million new bugs to
> accommodate a handful of corner cases.
>
> Which brings us back to ...
>
> Only the top (total) and bottom (null constant) patterns match null. Guy is
> happy again. (And the crowd goes wild.)
Nice bit of reasoning, and I am indeed happy again. (I was hoping I would have
to write “switch?” exactly once in my life. Well, this makes twice. But this
isa much better solution.)
—Guy
