Records -- current status
brian.goetz at oracle.com
Fri Mar 16 18:55:19 UTC 2018
There are a number of potentially open details on the design for
records. My inclination is to start with the simplest thing that
preserves the flexibility and expectations we want, and consider opening
up later as necessary.
One of the biggest issues, which Kevin raised as a must-address issue,
is having sufficient support for precondition validation. Without
foreclosing on the ability to do more later with declarative guards, I
think the recent construction proposal meets the requirement for
lightweight enforcement with minimal or no duplication. I'm hopeful
that this bit is "there".
Our goal all along has been to define records as being “just macros” for
a finer-grained set of features. Some of these are motivated by
boilerplate; some are motivated by semantics (coupling semantics of API
elements to state.) In general, records will get there first, and then
ordinary classes will get the more general feature, but the default
answer for "can you relax records, so I can use it in this case that
almost but doesn't quite fit" should be "no, but there will probably be
a feature coming that makes that class simpler, wait for that."
Some other open issues (please see my writeup at
http://cr.openjdk.java.net/~briangoetz/amber/datum.html for reference),
and my current thoughts on these, are outlined below. Comments welcome!
- Extension. The proposal outlines a notion of abstract record, which
provides a "width subtyped" hierarchy. Some have questioned whether
this carries its weight, especially given how Scala doesn't support
case-to-case extension (some see this as a bug, others as an existence
proof.) Records can implement interfaces.
- Concrete records are final. Relaxing this adds complexity to the
equality story; I'm not seeing good reasons to do so.
- Additional constructors. I don't see any reason why additional
constructors are problematic, especially if they are constrained to
delegate to the default constructor (which in turn is made far simpler
if there can be statements ahead of the this() call.) Users may find the
lack of additional constructors to be an arbitrary limitation (and
they'd probably be right.)
- Static fields. Static fields seem harmless.
- Additional instance fields. These are a much bigger concern. While
the primary arguments against them are of the "slippery slope" variety,
I still have deep misgivings about supporting unrestricted non-principal
instance fields, and I also haven't found a reasonable set of
restrictions that makes this less risky. I'd like to keep looking for a
better story here, before just caving on this, as I worry doing so will
end up biting us in the back.
- Mutability and accessibility. I'd like to propose an odd choice
here, which is: fields are final and package (protected for abstract
records) by default, but finality can be explicitly opted out of
(non-final) and accessibility can be explicitly widened (public).
- Accessors. Perhaps the most controversial aspect is that records
are inherently transparent to read; if something wants to truly
encapsulate state, it's not a record. Records will eventually have
pattern deconstructors, which will expose their state, so we should go
out of the gate with the equivalent. The obvious choice is to expose
read accessors automatically. (These will not be named getXxx; we are
not burning the ill-advised Javabean naming conventions into the
language, no matter how much people think it already is.) The obvious
naming choice for these accessors is fieldName(). No provision for
write accessors; that's bring-your-own.
- Core methods. Records will get equals, hashCode, and toString.
There's a good argument for making equals/hashCode final (so they can't
be explicitly redeclared); this gives us stronger preservation of the
data invariants that allow us to safely and mechanically snapshot /
serialize / marshal (we'd definitely want this if we ever allowed
additional instance fields.) No reason to suppress override of
toString, though. Records could be safely made cloneable() with
automatic support too (like arrays), but not clear if this is worth it
(its darn useful for arrays, though.) I think the auto-generated
getters should be final too; this leaves arrays as second-class
components, but I am not sure that bothers me.
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