JEP proposed to target JDK 12: 325: Switch Expressions (Preview) (original) (raw)
Stephen Colebourne scolebourne at joda.org
Thu Aug 23 12:16:25 UTC 2018
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As I have expressed previously on amber-dev, I find the design adopted here to be overly complex and with a poor syntax choice. There are aspects to like, and the proposal is better than earlier versions. I also acknowledge that this is a proposal to create a preview language feature, however in all likelihood, there will be no further changes to the syntax/design once integrated as a preview.
Some specific objections:
(FYI, "classic" uses colon and has fallthrough-by-default, "enhanced" uses arrow -> and does not allow fall-through).
the original goal was to add one new language feature (expression switch), yet the solution adds three (classic expression switch, enhanced statement switch & enhanced expression switch). This unnecessarily complicates Java for little gain IMO.
all the evidence I've seen suggests that fallback in switch is very rare, and I'd strongly argue that most developers consider fallthrough-by-default to be a poor feature of Java. Adding a new language feature (classic expression switch) which adds more fallthrough-by-default is a mistake IMO. It is likely that style guides and best practice will recommend against using classic expression switch. I do not accept that consistency ("it makes a 2x2 grid") is a sufficient reason to add a new language feature that will effectively be dead on arrival.
the use of -> as the separator in enhanced switch is undesirable IMO. Objections to the syntax have also been made by some expert group members. The particular problem is that it means something different to the use of -> in lambda expressions. This can result in confusing semi-puzzlers, such as "case RED -> YELLOW -> 6";.
the vast majority of existing switch statements would be more safely expressed using the enhanced form. As such, the enhanced form effectively replaces the classic form in Java - its that much better that classic statement switch will just be abandoned by developers IMO. Given this and the pointlessness of adding classic expression switch, the argument for using the keyword
switchfor the enhanced forms is weak IMO. At my presentations on the topic, the vast majority of developers have come to the conclusion that the enhanced form should use a different keyword, for examplematch, notswitch. Doing so allows the colon to be used instead of the arrow:
var name = match (trafficLight) { case RED: "Red"; case YELLOW: "Yellow"; case GREEN: "Green"; };
- Adding a new keyword/symbol for the expression form (and potentially declaring it an ExpressionStatement instead of having a separate statement form) would IMO be a simpler approach to the problem space, potentially adding just 1 new form rather than 3 as proposed by the JEP.
In summary, the world will not end if the JEP 325 progresses, and this
proposal is much better than previous ones. However I believe that by
taking the route of expanding switch rather than using a different
keyword/symbol the result has been greater complexity than is
necessary to meet the actual goal of "expression switch".
thanks Stephen
On 17 August 2018 at 18:44, <mark.reinhold at oracle.com> wrote:
The following JEP is proposed to target JDK 12:
325: Switch Expressions (Preview) http://openjdk.java.net/jeps/325 Feedback on this proposal is more than welcome, as are reasoned objections. If no such objections are raised by 23:00 UTC on Friday, 24 August, or if they’re raised and then satisfactorily answered, then per the JEP 2.0 process proposal [1] I’ll target this JEP to JDK 12. - Mark
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