----- Mail original -----
> De: "Jochen Theodorou" <[email protected]>
> À: "dev" <[email protected]>
> Envoyé: Jeudi 8 Mars 2018 19:51:57
> Objet: Re: [GEP] Switch expressions syntax from Java 11 or 12 (perhaps)
> On 08.03.2018 17:34, Remi Forax wrote:
> [...]
>>>> int accumulator = 0
>>>> LOOP: for (T element : someList) {
>>>> accumulator += switch (element.type) {
>>>> case PLUS_ONE -> +1;
>>>> case MINUS_ONE -> -1;
>>>> case ERROR:
>>>> break LOOP;
>>>> case default -> 0
>>>> }
>>>> }
>>>
>>> with the idea that element.type is an enum... But my problem is with
>>> break LOOP. Is LOOP the label LOOP, or is it a constant/variable?
>>
>> this will not compile, because you can not use return or break/continue
>> inside
>> an expression switch.
>
> you mean I cannot use the normal break/continue inside an expression
> switch, because this example
>
>> int result = switch (s) {
>> case "Foo" -> 1;
>> case "Bar" -> 2;
>> default:
>> System.out.println("Neither Foo nor Bar, hmmm...");
>> break 3;
>> }
>
> is straight from the JEP 325 page. And that was actually my point, this
> overlap, but redefinition of break.
yes, you can not use the normal break/containue inside an expression,
so break foo; in an expression switch has only one meaning.
> break is what makes the switch-case ugly for most people in the first place.
> Giving that special treatment like this looks off to me.
using break here is ugly, i agree with you but we (the amber EG) are not able
to find a better solution,
if you know a better way to handle local return of a value in the case of a
switch, please help us.
>
>>> If they need the break only to break out of the switch, then why not
>>> capitalize on lambdas ?
>>
>> it was the first idea :)
>> but lambda can capture effectively final local variable, when a case in an
>> expression switch acts more like a routine so you can access any variables
>> with
>> no restriction. So re-using the same syntax for two different semantics is
>> not
>> a good idea.
>
> A problem you would not have in Groovy, because we don´t do the
> capturing like Java.
It's not like in java but you use a box so the perf model is still not the same
between a captured variable and a plain old variable in Groovy.
>
>> The idea is that the expression switch should be used when it's a simple
>> switch
>> and the C switch with all it's subtle behaviors if the control flow is more
>> complex,
>> exactly like you use for(:) if it's a simple loop and for(;;) if it's a more
>> complex one.
>
> maybe, but I still think it does not make so much sense on its own. With
> a step-by-step approach of getting features in I can understand this,
> but then it means to decide to use this here, before deciding the
> details of pattern matching... and that sounds wrong.
no, the Java part of the pattern matching is mostly decided (there is still
corner cases like how re-starting when a guard fails).
The pattern matching is delayed more because
1/ the VM support is not there, part will be in the jdk 11, but how to specify
a sealed interface to avoid to have to specify a default when you have cover
all the cases is still open.
2/ the support of de-constructor (extractor) to keep encapsulation without
allocation in the middle of the pattern matching requires:
2a/ value types, good progress have been made on the valhalla side, a
release of value types without changing generics (which is a major PITA) may be
a temporary solution.
2b/ another temporary solution is to make pattern matching only working on
records (data classes).
The only part of the pattern matching which is ugly IMO is how to specify a
"local return" of a value in a case with several instructions.
Now, releasing an expression switch first has several advantages:
- allow to align the old switch with the pattern matching switch by modernizing
the behavior of the old switch with respect to null.
- it fixes several bugs of the current switch implementation:
- having to recompile the switch when you change the enum
- the switch on string being slow when the number of cases is small
- reduce the switch footprint in term of bytecodes
- and obviously allow to use a switch that "returns" an expression.
>
> bye Jochen
regards,
Rémi
