[External] : Re: Pattern features for next iteration

[Brian Goetz]

On 1/20/2021 1:40 AM, Suminda Sirinath Salpitikorala Dharmasena wrote:
 > Can we accomodate something like:
 >
 >    if (strarr instanceof String[] {var head, var... tail} wholestrarr)
 >
 > or
 >
 >    if (strarr instanceof String[] {var head, var... tail})
 >
 > or
 >
 >    if (strarr instanceof String[] {String head, String... tail} 
wholestrarr)
 >
 > or
 >
 >    if (strarr instanceof String[] {String head, String... tail})
 >

I believe all of these questions were covered in the discussion, at 
least indirectly.

It is likely you'll be able to bind something to the whole array:

     String[] { var a, var b } arrayBinding

It is *unlikely* you'll be able to bind something to the tail, for a 
number of reasons.  (Including: there's no reasonable reprsentation for 
the tail of an array; the main reason to want to do this is to process 
the array by recursion, but without tail-call elimination, this is also 
unlikely to be satisfying.) Doing so would let you *write* the code you 
think you want to write, but that code would then be slow and unreliable.

> Also need to consider situations like:
>
>  if (true || (strarr instanceof String[] wholestrarr)) {
>     // what is wholestrarr here? Is it null? Is it a ClassCastException?
>  }

This is covered in the flow scoping rules.  At this point, wholestrarr 
would not be definitely assigned, so it is not in scope, and you can't 
use it.  You can only use bindings where it is provable that they matched.

> is this similar to
>
>  if (true || (strarr instanceof String[])) {
>   String[] wholestrarr = (String[]) strarr;
>  }

No; the latter compiles, the former would not.

> What about:
>
>  if ((arr instanceof int[] intarr) || (arr instanceof String[] strarr)) {
>     // what is intarr / strarr here? Is one of them null? Is it 
> a ClassCastException?
>  }

By the flow scoping rules (laid out in the "Pattern Matching: Semantics" 
document), neither is in scope here.