We need more keywords, captain!

Remi Forax forax at univ-mlv.fr
Thu Jan 17 09:45:24 UTC 2019

I think i prefer break-with,
the problem of break-return is that people will write it break return without the hyphen, break return is in my opinion too close to return if you read the code too fast and a break return without a value means nothing unlike a regular return.

I like break-with because it's obvious that you have to say with what value you want to break, which is exactly the issue we have with the current break syntax.

So i vote for break-with instead of break,
as Brian said, the expression switch is currently a preview feature of 12 so we can still tweak the syntax a bit.


----- Mail original -----
> De: "Guy Steele" <guy.steele at oracle.com>
> À: "Brian Goetz" <brian.goetz at oracle.com>
> Cc: "amber-spec-experts" <amber-spec-experts at openjdk.java.net>
> Envoyé: Mardi 8 Janvier 2019 18:23:36
> Objet: Re: We need more keywords, captain!

> Actually, even better than `break-with` would be `break-return`.  It’s clearly a
> kind of `break`, and also clearly a kind of `return`.
> I think maybe this application alone has won me over to the idea of hyphenated
> keywords.
> (Then again, for this specific application we don’t even need the hyphen; we
> could just write `break return v;`.)
> —Guy
>> On Jan 8, 2019, at 12:35 PM, Brian Goetz <brian.goetz at oracle.com> wrote:
>> When discussing this today at our compiler meeting, we realized a few more
>> places where the lack of keywords produce distortions we don't even notice.  In
>> expression switch, we settled on `break value` as the way to provide a value
>> for a switch expression when the shorthand (`case L -> e`) doesn't suffice, but
>> this was painful for everyone.  It's painful for users because there's now work
>> required to disambiguate whether `break foo` is a labeled break or a value
>> break; it was even more painful to specify, because a new form of abrupt
>> completion had to be threaded through the spec.
>> Being able to call this something like `break-with v` (or some other derived
>> keyword) would have made this all a lot simpler. (BTW, we can still do this,
>> since expression-switch is still in preview.)
>> Moral of the story: even just a few minutes of brainstorming led us to several
>> applications of this approach that we hadn't seen a few days ago.
>> On 1/8/2019 10:22 AM, Brian Goetz wrote:
>>> This document proposes a possible move that will buy us some breathing room in
>>> the perpetual problem where the keyword-management tail wags the
>>> programming-model dog.
>>> ## We need more keywords, captain!
>>> Java has a fixed set of _keywords_ (JLS 3.9) which are not allowed to
>>> be used as identifiers.  This set has remained quite stable over the
>>> years (for good reason), with the exceptions of `assert` added in 1.4,
>>> `enum` added in 5, and `_` added in 9.  In addition, there are also
>>> several _reserved identifiers_ (`true`, `false`, and `null`) which
>>> behave almost like keywords.
>>> Over time, as the language evolves, language designers face a
>>> challenge; the set of keywords imagined in version 1.0 are rarely
>>> suitable for expressing all the things we might ever want our language
>>> to express.  We have several tools at our disposal for addressing this
>>> problem:
>>>  - Eminent domain.  Take words that were previously identifiers, and
>>>    turn them into keywords, as we did with `assert` in 1.4.
>>>  - Recycle.  Repurpose an existing keyword for something that it was
>>>    never really meant for (such as using `default` for annotation
>>>    values or default methods).
>>>  - Do without.  Find a way to pick a syntax that doesn't require a
>>>    new keyword, such as using `@interface` for annotations instead of
>>>    `annotation` -- or don't do the feature at all.
>>>  - Smoke and mirrors.  Create the illusion of context-dependent
>>>    keywords through various linguistic heroics (restricted keywords,
>>>    reserved type names.)
>>> In any given situation, all of these options are on the table -- but
>>> most of the time, none of these options are very good.  The lack of
>>> reasonable options for extending the syntax of the language threatens
>>> to become a significant impediment to language evolution.
>>> #### Why not "just" make new keywords?
>>> While it may be legal for us to declare `i` to be a keyword in a
>>> future version of Java, this would likely break every program in the
>>> world,  since `i` is used so commonly as an identifier.  (When the
>>> `assert` keyword was added in 1.4, it broke every testing framework.)
>>> The cost of remediating the effect of such incompatible changes varies
>>> as well; invalidating a name choice for a local variable has a local
>>> fix,  but invalidating the name of a public type or an interface
>>> method might well be fatal.
>>> Additionally, the keywords we're likely to want to reclaim are often
>>> those that are popular as identifiers (e.g., `value`, `var`,
>>> `method`), making such fatal collisions more likely.  In some cases,
>>> if the keyword candidate in question is sufficiently rarely used as an
>>> identifier, we might still opt to take that source-compatibility hit
>>> -- but names that are less likely to collide (e.g.,
>>> `usually_but_not_always_final`) are likely not the ones we want in our
>>> language. Realistically, this is unlikely to be a well we can go to
>>> very often, and the bar must be very high.
>>> #### Why not "just" live with the keywords we have?
>>> Reusing keywords in multiple contexts has ample precedent in
>>> programming languages, including Java.  (For example, we (ab)use `final`
>>> for "not mutable", "not overridable", and "not extensible".)
>>> Sometimes, using an existing keyword in a new context is natural and
>>> sensible, but usually it's not our first choice.  Over time, as the
>>> range of demands we place on our keyword set expands, this may well
>>> descend into the ridiculous; no one wants to use `null final` as a way
>>> of negating finality.  (While one might think such things are too
>>> ridiculous to consider, note that we received serious-seeming
>>> suggestions during JEP 325 to use `new switch` to describe a switch
>>> with different semantics.  Presumably to be followed by `new new
>>> switch` in ten years.)
>>> Of course, one way to live without making new keywords is to stop
>>> evolving the language entirely.  While there are some who think this
>>> is a fine idea, doing so because of the lack of available tokens would
>>> be a silly reason. We are convinced that Java has a long life ahead of
>>> it, and developers are excited about new features that enable to them
>>> to write more expressive and reliable code.
>>> #### Why not "just" make contextual keywords?
>>> At first glance, contextual keywords (and their friends, such as
>>> reserved type identifiers) may appear to be a magic wand; they let us
>>> create the illusion of adding new keywords without breaking existing
>>> programs.  But the positive track record of contextual keywords hides
>>> a great deal of complexity and distortion.
>>> Each grammar position is its own story; contextual keywords that might
>>> be used as modifiers (e.g., `readonly`) have different ambiguity
>>> considerations than those that might be use in code (e.g., a `matches`
>>> expression).  The process of selecting a contextual keyword is not a
>>> simple matter of adding it to the grammar; each one requires an
>>> analysis of potential current and future interactions.  Similarly,
>>> each token we try to repurpose may have its own special
>>> considerations;  for example, we could justify the use of `var` as a
>>> reserved type name  because because the naming conventions are so
>>> broadly adhered to.  Finally, the use of contextual keywords in
>>> certain  syntactic positions can create additional considerations for
>>> extending the syntax later.
>>> Contextual keywords create complexity for specifications, compilers,
>>> and IDEs.  With one or two special cases, we can often deal well
>>> enough, but if special cases were to become more pervasive, this would
>>> likely result in more significant maintenance costs or bug tail. While
>>> it is easy to dismiss this as “not my problem”, in reality, this is
>>> everybody’s problem. IDEs often have to guess whether a use of a
>>> contextual keyword is a keyword or identifier, and it may not have
>>> enough information to make a good guess until it’s seen more input.
>>> This results in worse user highlighting, auto-completion, and
>>> refactoring abilities — or worse.  These problems quickly become
>>> everyone's problems.
>>> So, while contextual keywords are one of the tools in our toolbox,
>>> they should also be used sparingly.
>>> #### Why is this a problem?
>>> Aside from the obvious consequences of these problems (clunky syntax,
>>> complexity, bugs), there is a more insidious hidden cost --
>>> distortion.  The accidental details of keyword management pose a
>>> constant risk of distortion in language design.
>>> One could consider the choice to use `@interface` instead of
>>> `annotation` for annotations to be a distortion; having a descriptive
>>> name rather than a funky combination of punctuation and keyword would
>>> surely have made it easier for people to become familiar with
>>> annotations.
>>> In another example, the set of modifiers (`public`, `private`,
>>> `static`, `final`, etc) is not complete; there is no way to say “not
>>> final” or “not static”. This, in turn, means that we cannot create
>>> features where variables or classes are `final` by default, or members
>>> are `static` by default, because there’s no way to denote the desire
>>> to opt out of it.  While there may be reasons to justify a locally
>>> suboptimal default anyway (such as global consistency), we want to
>>> make these choices deliberately, not have them made for us by the
>>> accidental details of keyword management. Choosing to leave out a
>>> feature for reasons of simplicity is fine; leaving it out because we
>>> don't have a way to denote the obvious semantics is not.
>>> It may not be obvious from the outside, but this is a constant problem
>>> in evolving the language, and an ongoing tax that we all pay, directly
>>> or indirectly.
>>> ## We need a new source of keyword candidates
>>> Every time we confront this problem, the overwhelming tendency is to
>>> punt and pick one of the bad options, because the problem only comes
>>> along every once in a while.  But, with the features in the pipeline, I
>>> expect it will continue to come along with some frequency, and I’d
>>> rather get ahead of it. Given that all of these current options are
>>> problematic, and there is not even a least-problematic move that
>>> applies across all situations, my inclination is to try to expand the
>>> set of lexical forms that can be used as keywords.
>>> As a not-serious example, take the convention that we’ve used for
>>> experimental features, where we prefix provisional keywords in
>>> prototypes with two underscores, as we did with `__ByValue` in the
>>> Valhalla prototype. (We commonly do this in feature proposals and
>>> prototypes, mostly to signify “this keyword is a placeholder for a
>>> syntax decision to be made later”, but also because it permits a
>>> simple implementation that is unlikely to collide with existing code.)
>>> We could, for example, carve out the space of identifiers that begin
>>> with underscore as being reserved for keywords. Of course, this isn’t
>>> so pretty, and it also means we'd have a mix of underscore and
>>> non-underscore keywords, so it’s not a serious suggestion, as much as
>>> an example of the sort of move we are looking for.
>>> But I do have a serious suggestion: allow _hyphenated_ keywords where
>>> one or more of the terms are already keywords or reserved identifiers.
>>> Unlike restricted keywords, this creates much less trouble for
>>> parsing, as (for example) `non-null` cannot be confused for a
>>> subtraction expression, and the lexer can always tell with fixed
>>> lookahead whether `a-b` is three tokens or one. This gives us a lot
>>> more room for creating new, less-conflicting keywords. And these new
>>> keywords are likely to be good names, too, as many of the missing
>>> concepts we want to add describe their relationship to existing
>>> language constructs -- such as `non-null`.
>>> Here’s some examples where this approach might yield credible
>>> candidates. (Note: none of these are being proposed here; this is
>>> merely an illustrative list of examples of how this mechanism could
>>> form keywords that might, in some particular possible future, be
>>> useful and better than the alternatives we have now.)
>>>   - `non-null`
>>>   - `non-final`
>>>   - `package-private` (the default accessibility for class members, currently not
>>>   denotable)
>>>   - `public-read` (publicly readable, privately writable)
>>>   - `null-checked`
>>>   - `type-static` (a concept needed in Valhalla, which is static relative to a
>>>   particular specialization of a class, rather than the class itself)
>>>   - `default-value`
>>>   - `eventually-final` (what the `@Stable` annotation currently suggests)
>>>   - `semi-final` (an alternative to `sealed`)
>>>   - `exhaustive-switch` (opting into exhaustiveness checking for statement
>>>     switches)
>>>   - `enum-class`, `annotation-class`, `record-class` (we might have chosen these
>>>      as an alternative to `enum` and `@interface`, had we had the option)
>>>   - `this-class` (to describe the class literal for the current class)
>>>   - `this-return` (a common request is a way to mark a setter or builder method
>>>     as returning its receiver)
>>> (Again, the point is not to debate the merits of any of these specific
>>> examples; the point is merely to illustrate what we might be able to do
>>> with such a mechanism.)
>>> Having this as an option doesn't mean we can't also use the other
>>> approaches when they are suitable; it just means we have more, and
>>> likely less fraught, options with which to make better decisions.
>>> There are likely to be other lexical schemes by which new keywords can
>>> be created without impinging on existing code; this one seems credible
>>> and reasonably parsable by both machines and humans.
>>> #### "But that's ugly"
>>> Invariably, some percentage of readers will have an immediate and
>>> visceral reaction to this idea.  Let's stipulate for the record that
>>> some people will find this ugly.  (At least, at first.  Many such
>>> reactions are possibly-transient (see what I did there?) responses
>>> to unfamiliarity.)

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