Treatment of nested 'break'

[Kevin Bourrillion]

On Thu, May 10, 2018 at 1:04 PM, Brian Goetz <brian.goetz at oracle.com> wrote:

This might not help, but perhaps think of it as a compound keyword; "break
> switch" is not "break with an argument of switch", but a multi-word keyword
> itself.
>

Well, then multi-word keywords are the thing that is novel and should
require very strong justification. :-)

But no. I've gotta disagree with this: If we tell users that they can do
"break while", "break for", "continue while", "continue for", and "break
switch", no one's mental model will be that those are five different new
keywords. There is a very obvious orthogonal separation between (first)
what to do and (second) at what scope to do it. That's what I meant by
"argument of".


Personally if I saw "break while", I think I'd immediately know what that
> means, and might even thank the author for being clear.
>

It's not bad.

Here's a quick tidbit from our codebase:

- Labeled break/continue is *unfathomably* rare -- a rough estimate is
0.07% of all break/continue statements. (Just to be clear, no, we don't
have any kind of style guide rule about avoiding the use of labels, or
static analysis complaining about it, etc.)

- And even among this small 0.3%, in virtually every one of them the
surrounding loops are of the same kind, so `break <keyword>` would not be
applicable.




On 5/10/2018 3:57 PM, Kevin Bourrillion wrote:
>
> I'm just going to say that naming a keyword as the argument of another
> keyword seems novel and unprecedented for Java, and as such I think should
> require pretty strong justification.
>
>
> On Thu, May 10, 2018 at 12:12 PM, Guy Steele <guy.steele at oracle.com>
> wrote:
>
>>
>> > On May 10, 2018, at 3:06 PM, Brian Goetz <brian.goetz at oracle.com>
>> wrote:
>> >
>> >
>> >> I think these are both valid explanations, with different outcomes,
>> but anyway it's fair to say that it would be confusing to have the latter
>> perspective and then try to explain how a value break can get past a
>> surrounding 'for' loop.
>> >
>> > One option is: you can't.  While I agree there is code that one might
>> like to write that is made cumbersome by this, it's a valid option, and not
>> one that is utterly terrible.
>> >
>> > Another option is to extend the break syntax along the lines of the
>> proposed continue syntax.  Suppose for every continuable construct x (for,
>> while, switch) we supported "continue x".  So for every breakable construct
>> y we could support "break y".  If a for loop were enclosed in an expression
>> switch, you could then say "break switch e".  Then
>> >
>> >    if (foo)
>> >        break;
>> >    else
>> >        break 3;
>> >
>> > becomes
>> >
>> >    if (foo)
>> >        break for;
>> >    else
>> >        break switch 3;
>> >
>> > and it is much more obvious what is going on.
>>
>> If we are willing to pile up keywords in that manner, an alternate
>> possibility is to spell a value-returning break in a different way:
>>
>>         return switch <expression>;
>>
>> Then your example can become (I have added the implicit context):
>>
>>         switch (…) { case 17 -> {
>>                 …
>>                 for (…) {
>>                    ...
>>                    if (foo)
>>                        break;
>>                    else
>>                        return switch 3;
>>                 … }
>>             … }
>>         … }
>>
>> The additional advantage of this approach is that it completely
>> eliminates the syntactic ambiguity between
>>
>>         break variableName;
>>
>> and
>>
>>         break labelName;
>>
>> Given that we think most occurrences of “return switch” (or “switch
>> return”, take your pick) will be abbreviated by -> anyway, this might be an
>> acceptable approach.
>>
>> You can then still choose to go ahead and also allow things like
>>
>>         break for;
>>         break switch;
>>         break while;
>>         continue for;
>>         continue switch;
>>
>> but that can be a separate decision; these become simply a way to avoid
>> using statement labels.
>>
>> —Guy
>>
>>
>
>
> --
> Kevin Bourrillion | Java Librarian | Google, Inc. | kevinb at google.com
>
>
>


-- 
Kevin Bourrillion | Java Librarian | Google, Inc. | kevinb at google.com
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