To summarize and highlight the second problem: patterns can fail. So any use
of patterns in the language must address how failure is to be handled.
> On Apr 7, 2020, at 2:29 PM, Brian Goetz <brian.go...@oracle.com> wrote:
>
> I'd like to take this up later, but let me read into the record a few points
> we've already discussed on this matter:
>
> - Yes, it is very tempting to like this syntax because in the degenerate
> case, where `Foo f` is a type pattern, it looks just like a declaration with
> an initializer, so much so that we can say "aha, you never knew Java had
> patterns from day 1!". But, ultimately this runs out of gas when patterns
> get more complicated, so be wary of being seduced by how natural it looks in
> the simplest case.
>
> Even the middle ground:
>
> Point(var x, var y) = p
>
> doesn't look much like a declaration, or worse
>
> Foo() = f
>
> looks like, well, who knows.
>
> - This really only works when the pattern is _total_ on the target type, so
> that the pattern provably applies (modulo null.) That can work for record
> deconstruction patterns but not always for other pattern types. Alternately,
> we can have a statement form "bind P = target else { ... }" for partial
> patterns, but then we lose the nice connection noted above with declarations.
>
>
>
>
> On 4/7/2020 2:20 PM, Remi Forax wrote:
>> There was discussions about the danger of only providing instanceof + the
>> deconstruction pattern and not a way to have the same kind of destructuring
>> when declaring local variables
>>
>> By example, instead of
>> Point p = ...
>> int x = p.x();
>> int y = p.y();
>>
>> one can write
>> Point p = ...
>> if (!(p instanceof Point(int x, int y)));
>>
>> I think we should restart those discussions because variables declarations
>> is like a third places where patterns can appear (instanceof and switch
>> being the other two).
>>
>> I don't really want to propose a syntax, so the next examples will be to
>> give an idea of the semantics
>> The idea is to allow patterns on the left side of an '='.
>> So
>> pattern = value;
>>
>> We already have
>> Point p2 = p; // the type pattern
>>
>> so if we expand it with the new patterns proposed for instanceof, we get
>> Point(int x, int y) = p; // the deconstruction pattern with explicit type
>> and
>> Point(var x, var y) = p; // the deconstruction pattern with var
>>
>> regards,
>> Rémi
>>
>>
>>
>>
>>
>>
>>
>
