JEP proposed to target JDK 12: 325: Switch Expressions (Preview)

Roman Kennke roman at
Tue Aug 28 22:13:04 UTC 2018

Thank you, Brian, for your explanations. It does help me to understand
the situation much better.


>> The few objections raised here are not new, having already been raised
>> and answered over on the amber-dev and amber-spec-experts lists.  I’ve
>> therefore targeted this JEP to JDK 12.
> As Mark says, the issues raised on this thread were already considered
> and discussed at some length during the design of the feature.  But, for
> the convenience of readers here, I will summarize for the record some of
> the reasons why these suggestions were not incorporated into the final
> design of the feature when they came around the first time.
> Stephen raised several specific objections, which I'll summarize as:
>  - You're increasing complexity by adding three features, one of which
> is silly, and which gives us more fallthrough rather than less, which is
> surely a move in the wrong direction!
>  - I don't love the syntax.
>  - You should have created a new syntactic form (hereafter, "snitch",
> for "new switch") and left classic switch for dead, rather than piling
> more complexity on existing switch, because existing switch is
> hopelessly tied to the dumb idea of fallthrough.  Die, fallthrough, die.
> Ben expressed general agreement, and specifically for the "you should
> have killed switch and made something new" position.  While I understand
> where these opinions come from, and we did consider these issues
> seriously, we concluded that abstracting the existing switch construct
> as we did was a better long-term path.
> Ben also raised the excellent point that language design is a balance
> between the interests of experienced developers and of newcomers -- a
> concern we struggle with in every single feature, and which also played
> into our decision here.
> The first sentiment, which I've seen expressed in a few places, relies
> on a somewhat tortured notion of "feature".  We have not added three
> features (enhanced statement switch, classic expression switch, enhanced
> expression switch), as much as added _two_ features, defined at a very
> different level:
>  - The ability for switch to be an expression or a statement
>  - The choice of classic case labels or enhanced case labels, where the
> latter are single consequent, fallthrough-free, and free of the
> confusing scoping of the classic switch block (essentially, fixing most
> of the things people complain about with respect to switch.)
>  These features work orthogonally, so that you can mix and match them
> freely, and I don't think anyone could reasonably object that either is
> frivolous or undesirable.  So while they do give rise to four
> combinations, these combinations are not features in themselves. 
> Defining the improvements to switch as orthogonal choices makes the
> design and implementation _simpler_, not more complex, as we can then
> reason about the semantics of a particular combination based on the
> semantics of simpler orthogonal primitives.
> The fact that some combinations of these features are more desirable
> than others is not evidence that a mistake was made.  This is often the
> natural consequence of providing simple primitives that can be combined
> orthogonally; some of the combinations are always going to be more
> sensible than others, and that's fine.  So while "classic expression
> switch" may seem silly, and will almost certainly be rarely used in
> practice, that doesn't mean that we should distort the language design
> to prevent it.  In fact, doing so would increase, not decrease, the
> complexity, as it introduces arbitrary constraints and special cases. 
> (As a possibly tortured analogy, while we might almost never write a
> combination of public setter and private getter, that's no reason to try
> and outlaw that combination.)
> As to syntax preferences ... because syntax is so deeply subjective,
> there is never going to be a syntax that satisfies everyone.  And,
> people being what they are, they tend to only complain about the things
> they disagree with, and are quiet about the things they agree with.  So
> any syntax decision (whatever it is) is going to be met with some degree
> of "I don't like it" or "I would have preferred X" -- but again, that
> isn't necessarily actionable or dispositive.  The syntax choice we made
> here is consistent with how similar things are done in similar languages
> -- and we think it will, perhaps after a few days of initial
> brain-retraining, be well-understood and accepted by Java developers.
> The final concern is a combination of pragmatism and language evolution
> philosophy -- whether it is better to abandon an existing feature and
> create a replacement, or to try and rehabilitate or generalize an
> existing feature.  While we asked ourselves many times whether switch
> could indeed be rehabilitated (not just in the context of this smaller
> feature, but in the context of the bigger feature arc of pattern
> matching), we found that, at each turn, it was more practical to
> rehabilitate it than one might have initially thought.  And we felt it
> better to build on the existing feature that is well understood and well
> documented, than to create a wholly new one, just because the old one
> was imperfect.
> Ben expressed concerns along the lines of "think of the students" with
> respect to the proliferation of switch forms, but the reality with a
> separate "snitch" form would most likely be worse.  Students would not
> be fully absolved of the need to learn "old switch", not only because
> they will encounter code that uses it, but because snitch would almost
> certainly be tilted towards the "happy cases", which, while common, are
> not universal.  And by un-anchoring snitch from switch, the differences
> would likely be more arbitrary.  So new students would encounter a
> language with two suspiciously similar constructs, but subtly different
> in harder-to-understand ways, and wonder why there are two ways to do
> it.  The burning hulk of "old switch" would lie forever on the roadside,
> but unable to be ever hauled away.  Its nice to think that we can invent
> the ideal "snitch" construct, and just leave "switch" in the past, but
> that's a fantasy.
> Now, if switch were truly broken, it might be different.  But for all
> the distaste of fallthrough-by-default, the existing switch construct is
> _not_ fundamentally broken; it's not just what we'd design now if we had
> a clean sheet of paper.  (BTW, fallthrough itself is not a problem, and
> sometimes is essential; the real mistake was fallthrough _by default_.) 
> Therefore, while we did consider whether it would be necessary to retire
> "switch" to pasture, we found no evidence that this was actually
> necessary -- and found that it was possible to enhance switch to support
> all the desired behaviors, rather than to abandon it, however tempting
> the latter course might initially seem.  Enhancing switch builds on the
> existing understanding of Java developers; abandoning it for snitch
> invalidates that understanding.
> At a meta level, this inclination -- "just abandon switch and build
> something better" -- illustrates a common temptation that is best
> avoided: over-rotating towards the desire to "fix" the mistakes of the
> past because they offend us, and because the current opportunity seems
> "the last chance" to right a wrong that's been bugging us for years. 
> These are natural motivations, but they are frequently siren-songs that
> lure us away from the path that calmer reason might otherwise have
> chosen.  Yes, it would have been better if switch were designed
> differently 20 years ago, but that was billions of programmer-hours and
> lines of code ago, and the calculus is very different with such a large
> existing base of code and user understanding.  So, while I understand
> why it feels like an "opportunity missed", the cost of "seizing" this
> opportunity was too great, and the benefit too small.  Its natural to
> regret that we can't fix the mistakes of the past, but in this case, it
> would have been the wrong choice.
> The preview mechanism will allow us to gather feedback on the feature
> from actual use, rather than theorizing from no examples, and
> potentially adjust the specification before final release if warranted. 
> So if any _new_ issues up come as a result of actual experience, we are
> happy to hear about them.

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