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<font size="4"><font face="monospace">The recent update of JEP 401
contained a number of refinements to the user model,
specifically, separating the primitive/reference distinction
into a number of smaller distinctions (e.g., nullable vs
non-nullable, optional vs required construction.) Overall this
has been a very positive step forward. <br>
<br>
We still have a need for the distinction between what we've been
calling B2 and B3; JEP 401 currently frames that in terms of
"construction is optional." This is a big step forward; indeed,
the key difference between them is whether the class _needs_ the
"variables start out as null, and all instances are created by
constructors" protection, or whether it admits the
lighter-weight initialization protocol of "there's a a standard
zero value, null-free variables are initialized to that" that
primitives enjoy today. (Note that B3 classes don't require
this lighter protocol, they merely enable it, much as primitives
all give you the option of boxing to get the full conservative
initialization protocol.)<br>
<br>
The idea of framing this as "construction is optional" is a good
one, but the expression of it proposed in JEP 401 feels "not
quite there". In this note I'll propose an alternative
presentation, but the main goal here is around terminology and
user model rather than syntax (so please keep the syntax
agitation to a reasonable level.)<br>
<br>
The key distinction between B2 and B3 is that B3 has a _default
value_ which the VM can summon at will. This enables
non-nullable heap variables to be flattened, because we can
initialize these the same way we initialize other fields and
array elements. Further, that default value is highly
constrained; it is a physical zero, the result of initializing
all fields to their default value. <br>
<br>
Flattening is of course a goal, but it is not something that
exists in the programming model -- its just an optimization.
What exists in the programming model is the default value, and
what this unlocks is the possibility for variables to be
_implicitly initializated_. Reference-typed variables today are
_explicitly initialized_; variables start out null and have to
be initialized with a constructed value. A class with a default
value has the option (opted in through null-exclusion) for its
variables to be implicitly initialized, which, like primitives,
means that they start out with a valid default value, and can be
further assigned to. <br>
<br>
Framed this way, the Valhalla performance story simplifies to:<br>
<br>
- Give up identity, get flattening on the stack;<br>
- Further give up explicit initialization, get flattening for
small objects on the heap;<br>
- Further give up atomicity, get flattening for larger objects
on the heap.<br>
<br>
Giving up explicit initialization entails both the class opting
out of explicit initialization, _and_ the variable opting out of
nullity. <br>
<br>
The key new terminology that comes out of this is implicit vs
explicit initialization. <br>
<br>
<br>
Syntactically, my preference is to indicate that the default
value can be summoned by giving a value class a _default
constructor_:<br>
<br>
</font></font><font size="4"><font face="monospace"><font size="4"><font face="monospace"> value class Complex { <br>
public final double re, im;<br>
<br>
public default Complex();<br>
}<br>
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A default constructor has no arguments, no body, no throws
clause, and implicitly initializes all fields to their default
values. Unlike identity classes, value classes don't get
constructions implicitly; a value class must declare at least
one constructor, default or otherwise. This replaces the idea
of "optional constructor", which is a negative statement about
construction ("but you don't have to call me"), with a more
direct and positive statement that there is a _default
constructor_ with the required properties. <br>
<br>
Note that this is similar to the existing concept of "default
constructor", which you get for free in an identity class if you
don't specify any constructors. It is possible we can unify
these features (and also with constructors in "agnostic"
abstract classes), but first let's work out what it would mean
in value classes, and see if we like it.<br>
<br>
In this model, a B3 class is just a value class with a default
constructor -> a default constructor means that you have the
choice of implicit or explicit initialization -> non-nullity
at the use site opts into implicit initialization -> B3! gets
flattening (for small layouts.) <br>
<br>
<br>
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