John pulled a nice Jedi-mind-trick on me, and pointed out that we
actually have two creation expressions for arrays:
new Foo[n]
new Foo[] { a0, .., an }
and that if we are dualizing, then we should have these two patterns:
new Foo[] { P0, ..., Pn } // matches arrays of exactly length N
new Foo[P] // matches arrays whose length match P
but that neither
new Foo[] { P, Q, ... } // previous suggestion
nor
new Foo[L] { P, Q } // current suggestion
correspond to either of those, which suggests that we may have
prematurely optimized the pattern form. The rational consequence of
this observation is to do
new Foo[] { P0, ..., Pn } // matches arrays of exactly length N
now (which is also the basis of varargs patterns), and once we have
constant patterns (which are kind of required for the second form to be
all that useful), come back for `Foo[P]`.
On 9/6/2022 5:11 PM, Brian Goetz 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.
