Disallowing break label (and continue label) inside an expression switch

Brian Goetz brian.goetz at oracle.com
Fri Mar 23 21:49:46 UTC 2018

Stepping back, I think there's two ways to look at this:

  - break expression and break label are totally different statements, 
that happen to be spelled similarly
  - break is the same statement all around, but just as return requires 
a value in a value-returning method and requires no value in a void 
method, the meaning of break must agree with the innermost breaky context.

I think the latter is far easier for users to reason about, while giving 
up relatively little flexibility.  So doing a "break label" in an 
e-switch is the same error as doing "return 3" in a void method.

On 3/23/2018 5:42 PM, Brian Goetz wrote:
>> The rest of it is about where to put the sharp edges:  Can I
>> break-e from a switch-e wherever I might consider doing return
>> from a method/lambda?  Or does break-e have extra restrictions
>> to prevent certain ambiguities?  Your answer is the latter.
> Right.  To avoid ambiguity with other breaky contexts, we let the 
> innermost breaky context determine the allowable break modes.
>> Speaking of ambiguities, should this be illegal, even
>> though under your rules it happens to be unambiguous?
>> Or is it just a puzzler we tolerate?
>>     e-switch {
>>         LABEL:
>>         s-switch {
>>            { int LABEL = 2; break LABEL; }
>>         }
>>     }
> It could be allowed (since you can't break-e from the switch), but it 
> seems safer to call it a compile error.  After all, you can always 
> alpha-rename the label.
>> Also the other way:
>>     LABEL:
>>     s-switch {
>>         e-switch { int LABEL = 2; break LABEL; }
>>     }
>> You want "break LABEL" to be immediately recognized as
>> either a break-l or a break-e.  The above cases seem to
>> make it hard to do so.  We could declare such code
>> pathological and demand a relabel in either case,
>> just as we declare local variable shadowing pathological
>> in most cases, demanding a renaming of one of the
>> variables.  Local variable shadowing is more likely
>> to occur than label shadowing, given that labels
>> are a rare construct, so maybe we just let the
>> above be a puzzler, rather than add a rule.
> Or, make it the same rule.  If in "break x", x could resolve as either 
> a label or an expression, call it an ambiguity.  (In either case, 
> depending on whether you rename the variable or the label, you could 
> then get a different error, which is "we don't serve that kind of 
> break round these parts.")  But I think its the same game either way.

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