Array patterns (and varargs patterns)

[Brian Goetz]

> Other languages have patent matching on head and tail of lists. Adding 
> this may mean there should be additional syntax.

I alluded to this in my mail, hoping that people wouldn't allow 
themselves to be distracted by this particular siren song:

<digression>
People are immediately going to ask "can I bind something to the 
remainder"; I think this is mostly an "attractive distraction", and 
would prefer to not have this dominate the discussion.
</digression>

The idiom of "match a list by matching (head, tail)", which works great 
in Lisp and Haskell, is that (a) the representation of lists in Lisp and 
Haskell is a cons-cell, and (b) these language have tail-call 
elimination, so that processing a list by recursion in this manner is 
natural, efficient, and doesn't SOE.  Java has neither of these, so 
grafting the illusion of this model when the runtime is not suited to it 
is just moving the problem one foot down the road, but ultimately is not 
going to be satisfying.  The performance will be poor, and we'll SOE on 
lists longer than a few tens of thousands of elements.

The bottom line here is that idioms from Language X are very much 
connected to _the rest of language X_.  Ignoring this rarely works out 
well.

> Another issue is if the array is shorter say 0 length and one has:
>
>        if (arr instanceof int[] { var a, var b, ... }) { ... }
>
> Will this be an ArrayIndexOutOfBoundException or another exception? If 
> it is a reference type `a` and `b` can be null.

No.  Pattern matching is inherently conditional; the match above says 
"does `arr` match this array pattern".  Matching the array pattern means 
being an array, _and_ having the right number of elements, _and_ having 
those elements match the nested patterns. Matching should never throw 
(though, once we allow users to write patterns that have code in them, 
it will be possible to do things like NPE, but those are bugs.)

>
>
> On Wed, 6 Jan 2021 at 15:31, Gavin Bierman <gavin.bierman at oracle.com 
> <mailto:gavin.bierman at oracle.com>> wrote:
>
>     This is a feature that other languages call an “as pattern”,
>     because you are adding a new pattern variable binding to another
>     pattern (Standard ML used “as”, Haskell uses “@“ but calls it an
>     as pattern). It’s on our list of things to consider; part of the
>     consideration is whether it merits special syntax along with the
>     parsing issues of not having special syntax.
>
>     Gavin
>
>     > On 6 Jan 2021, at 05:44, Suminda Sirinath Salpitikorala
>     Dharmasena <sirinath1978m at gmail.com
>     <mailto:sirinath1978m at gmail.com>> wrote:
>     >
>     > Can we have something like:
>     >
>     >        if (arr instanceof String[] { var a, var b, ... }
>     stringArray) {
>     > ... }
>     >
>     > `stringArray` is optional but if present this will referrer to
>     the whole
>     > array.
>     >
>     >>
>