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.
