Records and annotations (was: Updated document on data classes and sealed types)

Sat Mar 9 17:56:51 UTC 2019

Only time to respond to the very last bit for now:

Interesting!  The criticism could be "that makes it look abstract", but I'm
not sure that's even a problem. If you had to put this information into an
interface implemented by the record, this is what you would write there
(and hope the annotation processor implements automatic
annotation-inheritance from supermethods), so it's pretty reasonable to
write here too.

On Sat, Mar 9, 2019 at 6:08 AM Brian Goetz <brian.goetz at oracle.com> wrote:

> An alternate is to allow the body to be left off entirely:
>
>      record R(int x, int y) {
>           @MyAnnotation
>           public boolean equals(Object o);
>      }
>
>
> On Mar 9, 2019, at 1:57 PM, Brian Goetz <brian.goetz at oracle.com> wrote:
>
> This raises a related question, which is: what if the author likes the
> default implementation, but wants to add more annotations (or Javadoc)?
>
> Currently, the story is decent for constructors; declaring an empty `Foo {
> }` constructor recreates the default behavior.  There is no equivalent for
> accessors, equals, hashCode, or toString.  We toyed with a “default.m()”
> syntax in the past:
>
>      record R(int x, int y) {
>           @MyAnnotation
>           public boolean equals(Object o) -> default.equals(o);
>      }
>
> which doesn’t seem so bad, and is surely better than trying (and maybe
> failing) to reproduce the default behavior imperatively.
>
>
> On Mar 9, 2019, at 12:47 PM, Brian Goetz <brian.goetz at oracle.com> wrote:
>
> This came up before, but we didn’t reach a conclusion.
>
> A record component is more than just the lower-level members (fields,
> accessors, ctor params) it gets desugared too.  So it seems reasonable that
> it be considered an annotatable program element, and that reflection expose
> directly the annotations on record components (separately from any
> annotations on the class members that may or may not derive from desugaring
> of records.)
>
> But, that still leaves the question of whether the desugaring should, or
> should not be, transparent to annotations.  My sense is that pushing
> annotations down to fields, ctor params, and accessors _seems_ friendly,
> but also opens a number of uncomfortable questions.
>
>  - Should we treat the cases where @A has a target of RECORD_COMPONENT,
> separately from the cases where it does not, such as, only push the
> annotation down to members when the target does not include
> RECORD_COMPONENT?  That is, is the desire to push down annotations based on
> “well, what if we want to apply a “legacy” annotation?  If so, this causes
> a migration compatibility issue; if someone adds RC to the targets list for
> @A, then when the record is recompiled, the location of the annotations
> will changed, possibly changing the behavior of frameworks that encounter
> the record.
>
>  - What if @A has a target set of { field, parameter }, but for some
> reason the user does _not_ want the annotation pushed down?  Tough luck?
> Redeclare the member without the annotation?
>
>  - If the user explicitly redeclares the member (ctor, accessor), what
> happens?  Do we still implicitly push down annotations from record
> components to the explicit member?  Will this be confusing when the source
> says “@B int x() -> x”, but reflection yields both @A and @B as annotations
> on x()?
>
> All of which causes me to back up and say: what is the motivation for
> pushing these down to implicit members, other than “general friendliness”?
>   Is this a migration strategy for migrating existing code to use records,
> without having to redeclare annotations on the members?  And if so, how
> useful is it really?  Will users want to throw the union of
> field/accessor/ctor parameter annotations on the record components just to
> gain compatibility with their existing code?
>
> My gut sense is that the stable solution is to make record component a new
> kind of target, and encourage frameworks to learn about these, rather than
> trying to fake out frameworks by emulating legacy behavior.
>
>
> On Mar 8, 2019, at 8:43 PM, Kevin Bourrillion <kevinb at google.com> wrote:
>
> Re: annotations,
>
> Doc says, "Record components constitute a new place to put annotations;
> we'll likely want to extend the @Target meta-annotation to reflect this."
>
> I'm sure we discussed this before, but I also expect to be able to put any
> METHOD-, FIELD- or PARAMETER-targeted annotation on a record component, and
> have that annotation appear to be present on the synthesized
> accessor/field/constructor-parameter. Is that sensible?
>
> (As for records themselves, I expect they are targeted with TYPE just as
> enums/interfaces/"plain old classes" (jeesh, is there any term that means
> the latter?).)
>
>
>
>
>
>
>
> On Fri, Mar 1, 2019 at 12:16 PM Brian Goetz <brian.goetz at oracle.com>
> wrote:
>
>> I've updated the document on data classes here:
>>
>>      http://cr.openjdk.java.net/~briangoetz/amber/datum.html
>>
>> (older versions of the document are retained in the same directory for
>> historical comparison.)
>>
>> While the previous version was mostly about tradeoffs, this version
>> takes a much more opinionated interpretation of the feature, offering
>> more examples of use cases of where it is intended to be used (and not
>> used).  Many of the "under consideration" flexibilities (extension,
>> mutability, additional fields) have collapsed to their more restrictive
>> form; while some people will be disappointed because it doesn't solve
>> the worst of their boilerplate problems, our conclusion is: records are
>> a powerful feature, but they're not necessarily the delivery vehicle for
>> easing all the (often self-inflicted) pain of JavaBeans.  We can
>> continue to explore relief for these situations too as separate
>> features, but trying to be all things to all classes has delayed the
>> records train long enough, and I'm convince they're separate problems
>> that want separate solutions.  Time to let the records train roll.
>>
>> I've also combined the information on sealed types in this document, as
>> the two are so tightly related.
>>
>> Comments welcome.
>>
>
>
> --
> Kevin Bourrillion | Java Librarian | Google, Inc. | kevinb at google.com
>
>
>
>
>

-- 
Kevin Bourrillion | Java Librarian | Google, Inc. | kevinb at google.com
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