A draft spec including the compromise strategy below is available at:
http://cr.openjdk.java.net/~gbierman/jep354-jls-20190524.html
<http://cr.openjdk.java.net/~gbierman/jep354-jls-20190524.html>
Comments welcomed!
Gavin
> On 22 May 2019, at 17:45, Brian Goetz <brian.go...@oracle.com> wrote:
>
> We’ve been drilling into the spec and implementation of yield as a contextual
> keyword. We have three possible strategies, all of which are specifiable and
> implementable, but with tradeoffs.
>
> The “dumb strategy” would be to say that `yield` is a keyword when it appears
> in the first position of a statement production (e.g., after an open brace or
> a semicolon.). This is simple to spec, and simple to implement, but it
> doesn’t so do well with variables named `yield`:
>
> yield++;
> yield = 3;
> if (foo)
> yield += 3;
> yield[3] = 4;
>
> The “smart strategy” says that `yield` is a keyword only within the context
> of the YieldStatement production; the rest of the time it is an identifier.
> This is also simple to spec, and does the right thing in all unambiguous
> cases, but requires unbounded lookahead, which compiler implementations may
> not like. The one ambiguous case is
>
> yield(e)
>
> which would match both YieldStatement and ExpressionStatement, and here we
> bias towards YieldStatement. Naked yield() invocations can qualify the
> invocation:
>
> this.yield(3)
> Thread.yield(4)
>
> The “compromise” strategy is like the smart strategy, except that it trades
> fixed lookahead for missing a few more method invocation cases. Here, we
> look at the tokens that follow the identifier yield, and use those to
> determine whether to classify yield as a keyword or identifier. (We’d choose
> identifier if it is an assignment op (=, +=, etc), left-bracket, dot, and a
> few others, plus a few two-token sequences (e.g., ++ and then semicolon),
> which is lookahead(2).
>
> The main difference between the compromise strategy and the smart strategy is
> the handling of method invocations that are not unary:
>
> yield(3, 4)
>
> In the smart strategy, we’d figure out that this is a method call; in the
> compromise strategy, we’d require qualification just as we do with the unary
> method.
>
> The compromise strategy misses some cases we could parse unambiguously, but
> also offers a simpler user model: always qualify invocations of methods
> called yield when used as expression statements. And it offers the better
> lookup behavior, which will make life easier for IDEs.
>
> So my recommendation here is the compromise strategy.
>
>> On May 21, 2019, at 10:50 AM, Tagir Valeev <amae...@gmail.com> wrote:
>>
>> I discussed this with colleagues and can confirm that for IntelliJ
>> IDEA parser it will be no problem to always consider yield as a
>> statement. At least it's much easier than to consider it as a
>> statement inside switchy blocks only.
>>
>> With best regards,
>> Tagir Valeev.
>>
>> On Tue, May 21, 2019 at 12:38 PM Tagir Valeev <amae...@gmail.com> wrote:
>>>
>>>> So does this (option B plus your No) mean that IDEs would tend to color
>>>> "yield" as a keyword (at the beginning of a statement) even if followed by
>>>> "("?
>>>
>>> My "No" was mostly against options C and D where symbol resolution
>>> affects the parse tree. Sorry if it wasn't clear from my message. When
>>> the context for the parsing is available inside the same Java file,
>>> it's usually ok. See the 'var' restricted keyword:
>>>
>>> var var = 10; // first is highlighted as type, second as local variable
>>> var = 20; // var is highlighted as local variable, despite it's at the
>>> beginning of a statement.
>>> var(1); // var is highlighted as a method call, despite it's at the
>>> beginning of a statement.
>>>
>>> We have no very big problems parsing this.
>>>
>>> With best regards,
>>> Tagir Valeev.
>>>
>>> On Tue, May 21, 2019 at 2:52 AM John Rose <john.r.r...@oracle.com> wrote:
>>>>
>>>> On May 20, 2019, at 8:24 AM, Tagir Valeev <amae...@gmail.com> wrote:
>>>>>
>>>>> Assuming that we agreed on 'yield' the option B seems the most
>>>>> attractive. A big No to context-specific parse tree. It's a complete pain
>>>>> to IDEs. Don't forget that IDE often deals with incomplete code, missing
>>>>> dependencies, etc., and still needs to provide reasonable highlighting
>>>>> and completion. Imagine that 'yield' method is available via import
>>>>> static Foo.* or superclass. In this case we don't want to look into other
>>>>> files to build a correct parse tree.
>>>>
>>>> So does this (option B plus your No) mean that IDEs would
>>>> tend to color "yield" as a keyword (at the beginning of a
>>>> statement) even if followed by "("?
>>>>
>>>> I suppose that would work. It's hard to predict what that
>>>> would feel like, but it's logical.
>>>>
>>>> — John
>
