<div dir="ltr"><div>Thank you both for your responses!</div><div><br></div><div>I understand that more diverse kinds of nested patterns interact with multiple other planned extensions and in order to better understand those interactions, nested patterns have been restricted to record patterns for now.</div><div><br></div><div>My question was less about constant patterns (or other unfinished extensions) and more about taking what is already possible at the top-level of a switch and allowing it also in nested pattern positions (probably caused by my hope that the notion of nested patterns could be extended incrementally with each new extension at the top-level). The example of refactoring a nested pattern into nested switches illustrates my intuition quite well: that nested patterns would have the same semantics as corresponding nested switches.</div><div><br></div><div>The idea of unifying top-level with nested possibilities raises the question whether null patterns or even guards should be available in nested patterns. But I restricted my question to CaseConstant (in the currently supported form) because that is the most obvious gap and it complicates refactorings of empty records into enum constants. Personally, I think nested guards could be quite confusing but there might be a case for nested null patterns.<br></div><div><br></div><div>Your summary helped me get an overview of previous considerations. I'll try to find more details in the archives and am looking forward to future discussions in the context of the mentioned JEPs.<br></div><div><br></div><div>Sebastian<br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, Nov 7, 2023 at 3:30 PM Brian Goetz <<a href="mailto:brian.goetz@oracle.com">brian.goetz@oracle.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div>
<font size="4"><font face="monospace">To add to this: there are
several categories of questions that we wanted to settle before
considering constant patterns (which would (almost?) complete
the rehabilitation of switch): <br>
<br>
- Syntactic. If `0` is both a literal and a pattern, then
locutions like `f(0)` can either be method invocations or
pattern matches. It was not obvious in the early days whether
such ambiguities would pose roadblocks for using patterns in
other contexts. This is seeming more viable now but we didn't
want to foreclose this path too early.<br>
<br>
- User perception. Related to the above, if it is not obvious
what `f(0)` means, even if the grammar can be parsed
unambiguously, it might make code harder to read. So we might
hold out for a syntax that is somewhat wordier but more
obvious. <br>
<br>
- Semantics. At the beginning, it was very much not clear (to
the point where our first ideas would have been pretty wrong)
what the semantics of matching a constant are. JEP 455 has been
working through those issues, so we believe we now have a firm
foundation for the meaning of `0` as a pattern.<br>
<br>
- Alignment with switch. The "obvious" meaning of constant
patterns may or may not align with the treatment of constants in
switch today; this needs to be worked through carefully,
otherwise refactoring a switch like:<br>
<br>
case Foo(k)<br>
case Foo(k')<br>
case Foo(k'')<br>
<br>
to a switch like<br>
<br>
case Foo(var x) { <br>
switch (x) { <br>
case k<br>
case k' <br>
case k''<br>
}<br>
}<br>
<br>
will have subtle asymmetries. <br>
<br>
So to sum this up, constant patterns are an "obvious" addition,
but which turn out to be very much non-obvious when you consider
all the connections with other language features and future
language directions. So we've been waiting for these to settle.<br>
</font></font><br>
<div>On 11/7/2023 4:37 AM, Gavin Bierman
wrote:<br>
</div>
<blockquote type="cite">
<pre>Hi Sebastian,
Yes, there has been some discussion about adding constant patterns if you look through the archive. There are interesting syntactic issues, but IIRC we have been waiting for the primitive type patterns feature to settle so we have a proper basis to deal with any type conversion questions. (For example, you write switch(new Box(b)) { case Box(0) -> … } where b is a byte and 0 is an integer constant…)
So, it’s on our radar :-)
Gavin
</pre>
<blockquote type="cite">
<pre>On 2 Nov 2023, at 11:13, Sebastian Fischer <a href="mailto:mail@sebfisch.de" target="_blank"><mail@sebfisch.de></a> wrote:
Hello.
I would like to improve my understanding of the interaction of two JEPs: Pattern matching for switch and Record patterns.
I am new to this list so might have missed previous discussion about this but could not find what I was looking for in the archives.
JEP 441 shows the following grammar for switch labels.
SwitchLabel:
case CaseConstant { , CaseConstant }
case null [, default]
case Pattern [ Guard ]
default
JEP 440 shows the following grammar for patterns.
Pattern:
TypePattern
RecordPattern
TypePattern:
LocalVariableDeclaration
RecordPattern:
ReferenceType ( [ PatternList ] )
PatternList :
Pattern { , Pattern }
As a consequence it is not possible to have case constants in the pattern list of a record pattern. For example consider the following definitions modeling (a simplified version of) boolean expressions.
sealed interface BoolExpr {
enum Constant implements BoolExpr { TRUE, FALSE }
record And(BoolExpr left, BoolExpr right) implements BoolExpr {}
default BoolExpr recursively(UnaryOperator<BoolExpr> transform) {
return transform.apply(switch (this) {
case Constant c -> c;
case And(var left, var right) -> new And(
left.recursively(transform),
right.recursively(transform)
);
});
}
default BoolExpr shortCircuit() {
return recursively(expr -> switch (expr) {
case And(var left, var unused)
when left == Constant.FALSE -> Constant.FALSE;
default ->
expr;
});
}
}
In the definition of `shortCircuit` (which does partial evaluation) we need to use a guard to check if the left argument of `And` is false.
Instead of using an enum, we can model constants as empty records as follows.
sealed interface Constant extends BoolExpr permits True, False {}
record True() implements Constant {}
record False() implements Constant {}
Now we can write `shortCircuit` using a nested record pattern.
default BoolExpr shortCircuit() {
return recursively(expr -> switch (expr) {
case And(False(), var unused) -> new False();
default -> expr;
});
}
It would be convenient to be able to use a nested pattern instead of a guard with the original definition using an enum for constants. Here is a hypothetical implementation of `shortCircuit` that is currently not supported.
default BoolExpr shortCircuit() {
return recursively(expr -> switch (expr) {
case And(Constant.FALSE, var unused) -> Constant.FALSE;
default -> expr;
});
}
What are potential problems with allowing case constants as nested patterns?
Kind regards,
Sebastian
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