Array patterns (and varargs patterns)

[Tagir Valeev]

Hello!

Honestly, to me this whole feature looks not very important. It's a
rare case in modern Java applications that business logic operates
with arrays directly. They are mostly used in low-level system code
where performance matters more than code elegance. Custom defined
named patterns for lists would be much more useful. Moreover, if named
patterns are supported, then array deconstruction could be implemented
in a library, without complicating the language specification (like `x
instanceof Arrays.of(String first, String next, String last)`).

With best regards,
Tagir Valeev.

On Tue, Sep 6, 2022 at 11:11 PM Brian Goetz <brian.goetz at oracle.com> wrote:
>
> We dropped this out of the record patterns JEP, but I think it is time to revisit this.
>
> The concept of array patterns was pretty straightforward; they mimic the nesting and exhaustiveness rules of record patterns, they are just a different sort of container for nested patterns.  And they have an obvious duality with array creation expressions.
>
> The main open question here was how we distinguish between "match an array of length exactly N" (where there are N nested patterns) and "match an array of length at least N".  We toyed with the idea of a "..." indicator to mean "more elements", but this felt a little forced and opened new questions.
>
> It later occurred to me that there is another place to nest a pattern in an array pattern -- to match (and bind) the length.  In the following, assume for sake of exposition that "_" is the "any" pattern (matches everything, binds nothing) and that we have some way to denote a constant pattern, which I'll denote here with a constant literal.
>
> There is an obvious place to put this (optional) pattern: in between the brackets.  So:
>
>     case String[1] { P }:
>                 ^ a constant pattern
>
> would match string arrays of length 1 whose sole element matches P.  And
>
>     case String[] { P, Q }
>
> would match string arrays of length exactly 2, whose first two elements match P and Q respectively.  (If the length pattern is not specified, we infer a constant pattern whose constant is equal to the length of the nested pattern list.)
>
> Matching a target to `String[L] { P0, .., Pn }` means
>
>     x instanceof String[] arr
>         && arr.length matches L
>         && arr.length >= n
>         && arr[0] matches P0
>         && arr[1] matches P1
>         ...
>         && arr[n] matches Pn
>
> More examples:
>
>     case String[int len] { P }
>
> would match string arrays of length >= 1 whose first element matches P, and further binds the array length to `len`.
>
>     case String[_] { P, Q }
>
> would match string arrays of any length whose first two elements match P and Q.
>
>     case String[3] { }
>                 ^constant pattern
>
> matches all string arrays of length 3.
>
>
> This is a more principled way to do it, because the length is a part of the array and deserves a chance to match via nested patterns, just as with the elements, and it avoid trying to give "..." a new meaning.
>
> The downside is that it might be confusing at first (though people will learn quickly enough) how to distinguish between an exact match and a prefix match.
>
>
>
>
> On 1/5/2021 1:48 PM, Brian Goetz wrote:
>
> As we get into the next round of pattern matching, I'd like to opportunistically attach another sub-feature: array patterns.  (This also bears on the question of "how would varargs patterns work", which I'll address below, though they might come later.)
>
> ## Array Patterns
>
> If we want to create a new array, we do so with an array construction expression:
>
>     new String[] { "a", "b" }
>
> Since each form of aggregation should have its dual in destructuring, the natural way to represent an array pattern (h/t to AlanM for suggesting this) is:
>
>     if (arr instanceof String[] { var a, var b }) { ... }
>
> Here, the applicability test is: "are you an instanceof of String[], with length = 2", and if so, we cast to String[], extract the two elements, and match them to the nested patterns `var a` and `var b`.   This is the natural analogue of deconstruction patterns for arrays, complete with nesting.
>
> Since an array can have more elements, we likely need a way to say "length >= 2" rather than simply "length == 2".  There are multiple syntactic ways to get there, for now I'm going to write
>
>     if (arr instanceof String[] { var a, var b, ... })
>
> to indicate "more".  The "..." matches zero or more elements and binds nothing.
>
> <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>
>
> Here's an example from the JDK that could use this effectively:
>
> String[] limits = limitString.split(":");
> try {
>     switch (limits.length) {
>         case 2: {
>             if (!limits[1].equals("*"))
>                 setMultilineLimit(MultilineLimit.DEPTH, Integer.parseInt(limits[1]));
>         }
>         case 1: {
>             if (!limits[0].equals("*"))
>                 setMultilineLimit(MultilineLimit.LENGTH, Integer.parseInt(limits[0]));
>         }
>     }
> }
> catch(NumberFormatException ex) {
>     setMultilineLimit(MultilineLimit.DEPTH, -1);
>     setMultilineLimit(MultilineLimit.LENGTH, -1);
> }
>
> becomes (eventually)
>
>     switch (limitString.split(":")) {
>         case String[] { var _, Integer.parseInt(var i) } -> setMultilineLimit(DEPTH, i);
>         case String[] { Integer.parseInt(var i) } -> setMultilineLimit(LENGTH, i);
>         default -> { setMultilineLimit(DEPTH, -1); setMultilineLimit(LENGTH, -1); }
>     }
>
> Note how not only does this become more compact, but the unchecked "NumberFormatException" is folded into the match, rather than being a separate concern.
>
>
> ## Varargs patterns
>
> Having array patterns offers us a natural way to interpret deconstruction patterns for varargs records.  Assume we have:
>
>     void m(X... xs) { }
>
> Then a varargs invocation
>
>     m(a, b, c)
>
> is really sugar for
>
>     m(new X[] { a, b, c })
>
> So the dual of a varargs invocation, a varargs match, is really a match to an array pattern.  So for a record
>
>     record R(X... xs) { }
>
> a varargs match:
>
>     case R(var a, var b, var c):
>
> is really sugar for an array match:
>
>     case R(X[] { var a, var b, var c }):
>
> And similarly, we can use our "more arity" indicator:
>
>     case R(var a, var b, var c, ...):
>
> to indicate that there are at least three elements.
>
>
>