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<font size="4"><font face="monospace">You're not wrong; having only
B3! is a much simpler feature, because (a) it is enforced by the
VM and (b) we don't have the split of "sometimes you can believe
the bang, sometimes you can't." (On the other hand, imagine the
cries of bloody murder when we tell people they can't have any
form of `String!`.) And yes, arrays are particularly
challenging. <br>
<br>
The document acknowledged to this in that it said we might start
with B3! only, but we also have to work out what our options are
for extending to other reference types. (As it turns out, the
anomalies are fairly close to, though not exactly the same as,
heap pollution with erased generics -- which is to say,
pollution still sucks, but its a pollution we're at least
somewhat familiar with.)<br>
<br>
I don't see where you get "erased bangs today foreclose on
enforced bangs tomorrow" (and if that's true, we also have no
path to generic specialization.)<br>
<br>
I think its a significant overstatement to say "syntax but not
semantics" or "only to have a more uniform syntax"; the concepts
and semantics are also the same, it is the runtime guarantees
that are different under some circumstances. Your concern is
valid, but don't overstate it. <br>
<br>
All of this is to say: your concerns are valid and we've been
struggling with them, but I think saying "no String! ever" is
also not a realistic position, so somewhere compromises will
have to be made, and our job is to find the right set of
compromises. <br>
<br>
<br>
<br>
</font></font><br>
<div class="moz-cite-prefix">On 6/3/2023 2:12 AM, Remi Forax wrote:<br>
</div>
<blockquote type="cite" cite="mid:2075843456.71765771.1685772779404.JavaMail.zimbra@univ-eiffel.fr">
<div style="font-family: arial, helvetica, sans-serif; font-size:
12pt; color: #000000">
<div>Hi all,<br>
</div>
<div>I am not convinced that adding the nullability annotations
to other types than the value types with a default value is a
good move,</div>
<div>it seems to shut the door to potential futures where being
non null is more strongly enforced by the VM.<br data-mce-bogus="1">
</div>
<div>If the goal of Valhalla is to introduce value types, I
think we are extending too much our reach here, making
decisions for Java we may regret later.<br data-mce-bogus="1">
</div>
<div><br data-mce-bogus="1">
</div>
<div>In understand the appeal of providing an unified view
regarding nullability but sadly as explained in this document,
the unification will only be true in term of syntax not in
term of semantics, only the value types with a default value
being reified, enforced by the VM.<br data-mce-bogus="1">
</div>
<div><br data-mce-bogus="1">
</div>
<div>As an example, the proposal Array 2.0 of John is still on
the table and proposes to add non null array at runtime, so
choosing to erase the nullability annotation now seems a bad
move now if in the future such kind of arrays are added to the
Java platform.<br data-mce-bogus="1">
</div>
<div><br data-mce-bogus="1">
</div>
<div>Obviously, we want to allow migration from/to identity type
and value type (or from value type without a default value to
a value type with a default value) so we have to specify both
at compile time and at runtime a semantics that allow that.
But I am seeing erasing nullability annotations in case of
identity type as a too easy shortcut.<br data-mce-bogus="1">
</div>
<div><br data-mce-bogus="1">
</div>
<div>I would prefer to live in a world where '!' is only
available on value type with a default value at compile time,
with the field and array creation being only enforced at
runtime only if the class is a value class with a default
value.<br data-mce-bogus="1">
</div>
<div>A world where adding an implicit constructor is a source
backward compatible move but the opposite is not and where the
VM ignores nullability attributes at runtime if the class is
not actually a value class with a default value so moving from
a value class with a default value to a value class without a
default value is a binary compatible move.<br data-mce-bogus="1">
</div>
<div><br data-mce-bogus="1">
</div>
<div>With the model above, we only have null pollution because
of separate compilations, especially, we keep the property
that when unboxing (i.e. the transition T to T!) null checking
is done by the VM so there is no null pollution.</div>
<div>Allowing more erasure only to have a more uniform syntax is
not appealing to me and seems worst if seen from the future.<br data-mce-bogus="1">
</div>
<div><br data-mce-bogus="1">
</div>
<div>regards,<br data-mce-bogus="1">
</div>
<div>Rémi</div>
<div><br>
</div>
<hr id="zwchr" data-marker="__DIVIDER__">
<div data-marker="__HEADERS__">
<blockquote style="border-left:2px solid
#1010FF;margin-left:5px;padding-left:5px;color:#000;font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt;"><b>From:
</b>"Brian Goetz" <a class="moz-txt-link-rfc2396E" href="mailto:brian.goetz@oracle.com"><brian.goetz@oracle.com></a><br>
<b>To: </b>"valhalla-spec-experts"
<a class="moz-txt-link-rfc2396E" href="mailto:valhalla-spec-experts@openjdk.java.net"><valhalla-spec-experts@openjdk.java.net></a><br>
<b>Sent: </b>Wednesday, May 31, 2023 8:37:34 PM<br>
<b>Subject: </b>Design document on nullability and value
types<br>
</blockquote>
</div>
<div data-marker="__QUOTED_TEXT__">
<blockquote style="border-left:2px solid
#1010FF;margin-left:5px;padding-left:5px;color:#000;font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt;"><font size="4"><font face="monospace">As we've hinted at, we've
made some progress refining the essential differences
between primitive and reference types, which has enabled
us to shed the `.val` / `.ref` distinction and lean more
heavily on nullability. The following document outlines
the observations that have enabled this current turn of
direction and some of its consequences. <br>
<br>
This document is mostly to be interpreted in the context
of the Valhalla journey, and so talks about where we
were a few months ago and where we're heading now. <br>
<br>
<br>
<br>
# Rehabilitating primitive classes: a nullity-centric
approach<br>
<br>
Over the course of Project Valhalla, we have observed
that there are two<br>
distinct groups of value types. We've tried stacking
them in various ways, but<br>
there are always two groups, which we've historically
described as "objects<br>
without identity" and "primitive classes", and which
admit different degrees of<br>
flattening. <br>
<br>
The first group, which we are now calling "value
objects" or "value classes",<br>
represent the minimal departure from traditional classes
to disavow object<br>
identity. The existing classes that are described as
"value-based", such as<br>
`Optional` or `LocalDate`, are candidate for migrating
to value classes. Such<br>
classes give up object identity; identity-sensitive
behaviors are either recast<br>
as state-based (such as for `==` and
`Objects::identityHashCode`) or partialized<br>
(`synchronized`, `WeakReference`), and such classes must
live without the<br>
affordances of identity (mutability, layout
polymorphism.) In return, they<br>
avoid being burdened by "accidental identity" which can
be a source of bugs, and<br>
gain significant optimization for stack-based values
(e.g., scalarization in<br>
calling convention) and other JIT optimizations. <br>
<br>
The second group, which we had been calling "primitive
classes" (we are now<br>
moving away from that term), are those that are more
like the existing<br>
primitives, such as `Decimal` or `Complex`. Where
ordinary value classes, like<br>
identity classes, gave rise to a single (reference)
type, these classes gave<br>
rise to two types, a value type (`X.val`) and a
reference type (`X.ref`). This<br>
pair of types was directly analogous to legacy
primitives and their boxes. These<br>
classes come with more restrictions and more to think
about, but are rewarded<br>
with greater heap flattening. This model -- after
several iterations -- seemed<br>
to meet the goals for expressiveness and performance: we
can express the<br>
difference between `int`-like behavior and
`Integer`-like behavior, and get<br>
routine flattening for `int`-like types. But the result
still had many<br>
imbalances; the distinction was heavyweight, and a
significant fraction of the<br>
incremental specification complexity was centered only
on these types. We<br>
eventually concluded that the source of this was trying
to model the `int` /<br>
`Integer` distinction directly, and that this
distinction, while grounded in<br>
user experience, was just not "primitive" enough. <br>
<br>
In this document, we will break down the characteristics
of so-called "primitive<br>
classes" into more "primitive" (and hopefully less
ad-hoc) distinctions. This<br>
results in a simpler model, streamlines the syntactic
baggage, and enables us to<br>
finally reunite with an old friend, null-exclusion
(bang) types. Rather than<br>
treating "value types" and "reference types" as
different things, we can treat<br>
the existing primitives (and the "value projection" of
user-defined primitive<br>
classes) as being restricted references, whose
restrictions enable the desired<br>
runtime properties. <br>
<br>
## Primitives and objects<br>
<br>
In a previous edition of _State of Valhalla_, we
outlined a host of differences<br>
between primitives and objects:<br>
<br>
| Primitives |
Objects |<br>
| ------------------------------------------ |
----------------------------------------- |<br>
| No identity (pure values) |
Identity |<br>
| `==` compares state | `==`
compares object identity |<br>
| Built-in | Declared
in classes |<br>
| No members (fields, methods, constructors) | Members
(including mutable fields) |<br>
| No supertypes or subtypes | Class and
interface inheritance |<br>
| Represented directly in memory |
Represented indirectly through references |<br>
| Not nullable |
Nullable |<br>
| Default value is zero | Default
value is null |<br>
| Arrays are monomorphic | Arrays
are covariant |<br>
| May tear under race |
Initialization safety guarantees |<br>
| Have reference companions (boxes) | Don't
need reference companions |<br>
<br>
Over many iterations, we have chipped away at this list,
mostly by making<br>
classes richer: value classes can disavow identity (and
thereby opt into<br>
state-based `==` comparison); the lack of members and
supertypes are an<br>
accidental restriction that can go away with declarable
value classes; we can<br>
make primitive arrays covariant with arrays of their
boxes; we can let some<br>
class declarations opt into non-atomicity under race.
That leaves the<br>
following, condensed list of differences: <br>
<br>
| Primitives |
Objects |<br>
| --------------------------------- |
----------------------------------------- |<br>
| Represented directly in memory | Represented
indirectly through references |<br>
| Not nullable |
Nullable |<br>
| Default value is zero | Default value is
null |<br>
| Have reference companions (boxes) | Don't need
reference companions |<br>
<br>
The previous approach ("primitive classes") started with
the assumption that<br>
this is the list of things to be modeled by the
value/reference distinction. In<br>
this document we go further, by showing that flattening
(direct representation)<br>
is derived from more basic principles around nullity and
initialization<br>
requirements, and perhaps surprisingly, the concept of
"primitive type" can<br>
disappear almost completely, save only for historical
vestiges related to the<br>
existing eight primitives. The `.val` type can be
replaced by restricted<br>
references whose restrictions enable the desired
representational properties. As<br>
is consistent with the goals of Valhalla, flattenability
is an emergent<br>
property, gained by giving up those properties that
would undermine<br>
flattenability, rather than being a linguistic concept
on its own.<br>
<br>
### Initialization<br>
<br>
The key distinction between today's primitives and
objects has to do with<br>
_initialization requirements_. Primitives are designed
to be _used<br>
uninitialized_; if we declare a field `int count`, it is
reliably initialized to<br>
zero by the JVM before any code can access it. This
initial value is a<br>
perfectly good default, and it is not a bug to read or
even increment this field<br>
before it has been explicitly assigned a value by the
program, because it has<br>
_already_ been initialized to a known good value by the
JVM. The zero value<br>
pre-written by the JVM is not just a safety net; it is
actually part of the<br>
programming model that primitives start out life with
"good enough" defaults.<br>
This is part of what it means to be a primitive type.<br>
<br>
Objects, on the other hand, are not designed for
uninitialized use; they must be<br>
initialized via constructors before use. The default
zero values written to an<br>
object's fields by the JVM typically don't necessarily
constitute a valid state<br>
according to the classes specification, and, even if it
did, is rarely a good<br>
default value. Therefore, we require that class
instances be initialized by<br>
their constructors before they can be exposed to the
rest of the program. To<br>
ensure that this happens, objects are referenced
exclusively through _object<br>
references_, which _can_ be safely used uninitialized --
because they reliably<br>
have the usable default value of `null`. (Some may
quibble with this use of<br>
"safely" and "usable", because null references are
fairly limited, but they do<br>
their limited job correctly: we can easily and safely
test whether a reference<br>
is null, and if we accidentally dereference a null
reference, we get a clear<br>
exception rather than accessing uninitialized object
state.) <br>
<br>
> Primitives can be safely used without explicit
initialization; objects cannot.<br>
> Object references are nullable _precisely because_
objects cannot be used<br>
> safely without explicit initialization. <br>
<br>
### Nullability<br>
<br>
A key difference between today's primitives and
references is that primitives<br>
are non-nullable and references are nullable. One might
think this was<br>
primarily a choice of convenience: null is useful for
references as a universal<br>
sentinel, and not all that useful for primitives (when
we want nullable<br>
primitives we can use the box classes -- but we usually
don't.) But the<br>
reality is not one of convenience, but of necessity:
nullability is _required_<br>
for the safety of objects, and usually _detrimental_ to
the performance of<br>
primitives.<br>
<br>
Nullability for object references is a forced move
because null is what is<br>
preventing us from accessing uninitialized object
state. Nullability for<br>
primitives is usually not needed, but that's not the
only reason primitives are<br>
non-nullable. If primitives were nullable, `null` would
be another state that<br>
would have to be represented in memory, and the costs
would be out of line with<br>
the benefits. Since a 64-bit `long` uses all of its bit
patterns, a nullable<br>
`long` would require at least 65 bits, and alignment
requirements would likely<br>
round this up to 128 bits, doubling memory usage. (The
density cost here is<br>
substantial, but it gets worse because most hardware
today does not have cheap<br>
atomic 128 bit loads and stores. Since tearing might
conflate a null value with<br>
a non-null value -- even worse than the usual
consequences of tearing -- this<br>
would push us strongly towards using an indirection
instead.) So<br>
non-nullability is a precondition for effective
flattening and density of<br>
primitives, and nullable primitives would involve giving
up the flatness and<br>
density that are the reason to have primitives in the
first place. <br>
<br>
> Nullability interferes with heap flattening.<br>
<br>
To summarize, the design of primitives and objects
implicitly stems from the<br>
following facts: <br>
<br>
- For most objects, the uninitialized (zeroed) state is
either invalid or not a<br>
good-enough default value;<br>
- For primitives, the uninitialized (zeroed) state is
both valid and a<br>
good-enough default value; <br>
- Having the uninitialized (zeroed) state be a
good-enough default is a<br>
precondition for reliable flattening;<br>
- Nullability is required when the the uninitialized
(zeroed) state is not a<br>
good-enough default; <br>
- Nullability not only has a footprint cost, but often
is an impediment to<br>
flattening.<br>
<br>
> Primitives exist in the first place because they
can be flattened to give us<br>
> better numeric performance; flattening requires
giving up nullity and<br>
> tolerance of uninitialized (zero) values.<br>
<br>
These observations were baked in to the language (and
other languages too), but<br>
the motivation for these decisions was then "erased" by
the rigid distinction<br>
between primitives and objects. Valhalla seeks to put
that choice back into the<br>
user's hands.<br>
<br>
### Getting the best of both worlds<br>
<br>
Project Valhalla promises the best of both worlds:
sufficiently constrained<br>
entities can "code like a class and work like an int."
Classes that give up<br>
object identity can get some of the runtime benefits of
primitives, but to get<br>
full heap flattening, we must embrace the two defining
characteristics of<br>
primitives described so far: non-nullability and safe
uninitialized use. <br>
<br>
Some candidates for value classes, such as `Complex`,
are safe to use<br>
uninitialized because the default (zero) value is a good
initial value. Others,<br>
like `LocalDate`, simply have no good default value
(zero or otherwise), and<br>
therefore need the initialzation protocol enabled by
null-default object<br>
references. This distinction in inherent to the
semantics of the domain; some<br>
domains simply do not have reasonable default value, and
this is a choice that<br>
the class author must capture when the code is written.
<br>
<br>
There is a long list of classes that are candidates to
be value classes; some<br>
are like `Complex`, but many are more like `LocalDate`.
The latter group can<br>
still benefit significantly from eliminating identity,
but can't necessarily get<br>
full heap flattening. The former group, which are most
like today's primitives,<br>
can get all the benefits, including heap flattening --
when their instances are<br>
non-null. <br>
<br>
### Declaring value classes<br>
<br>
As in previous iterations, a class can be declared as as
_value class_:<br>
<br>
```<br>
value class LocalDate { ... }<br>
```<br>
<br>
A value class gives up identity and its consequences
(e.g., mutability) -- and<br>
that's it. The resulting `LocalDate` type is still a
reference type, and<br>
variables of type `LocalDate` are still nullable.
Instances can get significant<br>
optimizations for on-stack use but are still usually
represented in the heap via<br>
indirections. <br>
<br>
### Implicitly constructible value classes<br>
<br>
In order to get the next group of benefits, a value
class must additionally<br>
attest that it can be used uninitialized. Because this
is a statement of how<br>
instances of this class come into existence, modeling
this as a special kind of<br>
constructor seems natural:<br>
<br>
```<br>
value class Complex { <br>
private int re;<br>
private int im;<br>
<br>
public implicit Complex();<br>
public Complex(int re, int im) { ... }<br>
<br>
...<br>
}<br>
```<br>
<br>
These two constructors say that there are two ways a
`Complex` instance comes<br>
into existence: the first is via the traditional
constructor that takes real and<br>
imaginary values (`new Complex(1.0, 1.0)`), and the
second is via the _implicit_<br>
constructor that produces the instance used to
initialize fields and array<br>
elements to their default values. That the implicit
constructor cannot have a<br>
body is a signal that the "zero default" is not
something the class author can<br>
fine-tune. A value class with an implicit constructor
is called an _implicitly<br>
constructible_ value class.<br>
<br>
Having an implicit constructor is a necessary but not
sufficient condition for<br>
heap flattening. The other required condition is that
variable that holds a<br>
`Complex` needs to be non-nullable. In the previous
iteration, the `.val` type<br>
was non-nullable for the same reason primitive types
were, and therefore `.val`<br>
types could be fully flattened. However, after several
rounds of teasing apart<br>
the fundamental properties of primitives and value
types, nullability has<br>
finally sedimented to a place in the model where a
sensible reunion between<br>
value types and non-nullable types may be possible. <br>
<br>
## Null exclusion <br>
<br>
Non-nullable reference types have been a frequent
request for Java for years,<br>
having been explored in `C#`, Kotlin, and Scala. The
goals of non-nullable<br>
types are sensible: richer types means safer programs.
It is a pervasive<br>
problem in Java libraries that we are not able to
express within the language<br>
whether a returned object reference might be null, or is
known never to be null,<br>
and programmers can therefore easily make wrong
assumptions about nullability. <br>
<br>
To date, Project Valhalla has deliberately steered clear
of non-nullable types<br>
as a standalone feature. This is not only because the
goals of Valhalla were too<br>
ambitious to burden the project with another ambitious
goal (though that is<br>
true), but for a more fundamental reason: the
assumptions one might make in a<br>
vacuum about the semantics of non-nullable types would
likely become hidden<br>
sources of constraints for the value type design, which
was already bordering on<br>
over-constrained. Now that the project has progressed
sufficiently, we are more<br>
confident that we can engage with the issue of null
exclusion.<br>
<br>
A _refinement type_ (or _restriction type_) is a type
that is derived from<br>
another type that excludes certain values from the
derived type's value set,<br>
such as "the non-negative integers". In the most general
form, a refinement type<br>
is defined by one or more predicates (Liquid Haskell and
Clojure Spec are<br>
examples of this); range types in Pascal are a more
constrained form of<br>
refinement type. Non-nullable types ("bang" types) can
similarly be viewed as a<br>
constrained form of refinement type, characterized by
the predicate `x != null`.<br>
(Note that the null-excluding refinement type `X!` of a
reference type is still<br>
a reference type.)<br>
<br>
Rather than saying that primitive classes give rise to
two types, `X.val` and<br>
`X.ref`, we can observe the the null-excluding type `X!`
of a<br>
implicitly-constructible value class can have the same
runtime characteristic as<br>
the `.val` type in the previous round. Both the
declaration-site property that<br>
a value class is implicitly constructible, and the
use-site property that a<br>
variable is null-excluding, are necessary to routinely
get flattening. <br>
<br>
Related to null exclusion is _null-adjunction_; this
takes a non-nullable type<br>
(such as `int`) or a type of indeterminate nullability
(such as a type variable<br>
`T` in a generic class that can be instantiated with
either nullable or<br>
non-nullable type parameters) and produces a type that
is explicitly nullable<br>
(`int?` or `T?`.) In the current form of the design,
there is only one place<br>
where the null-adjoining type is strictly needed -- when
generic code needs to<br>
express "`T`, but might be null. The canonical example
of this is `Map::get`;<br>
it wants to wants to return `V?`, to capture the fact
that `Map` uses `null` to<br>
represent "no mapping".<br>
<br>
For a given class `C`, the type `C!` is clearly
non-nullable, and the type `C?`<br>
is clearly nullable. What of the unadorned name `C`?
This has _unspecified_<br>
nullability. Unspecified nullability is analogous to
raw types in generics (we<br>
could call this "raw nullability"); we cannot be sure
what the author had in<br>
mind, and so must find a balance between the desire for
greater null safety and<br>
tolerance of ambiguity in author intent.<br>
<br>
Readers who are familiar with explicitly nullable and
non-nullable types in<br>
other languages may be initially surprised at some of
the choices made regarding<br>
null-exclusion (and null-adjunction) types here. The
interpretation outlined<br>
here is not necessarily the "obvious" one, because it is
constrained both by the<br>
needs of null-exclusion, of Valhalla, and the
migration-compatibility<br>
constraints needed for the ecosystem to make a
successful transition to types<br>
that have richer nullability information. <br>
<br>
While the theory outlined here will allow all class
types to have a<br>
null-excluding refinement type, it is also possible that
we will initially<br>
restrict null-exclusion to implicitly constructible
value types. There are<br>
several reasons to consider pursuing such an incremental
path, including the<br>
fact that we will be able to reify the non-nullability
of implicitly<br>
constructible value types in the JVM, whereas the
null-exclusion types of other<br>
classes such as `String` or of ordinary value classes
such as `LocalDate` would<br>
need to be done through erasure, increasing the possible
sources of null<br>
polluion. <br>
<br>
### Goals<br>
<br>
We adopt the following set of goals for adding
null-excluding refinement types: <br>
<br>
- More complete unification of primitives with classes;<br>
- Flatness is an emergent property that can derive from
more basic semantic<br>
constraints, such as identity-freedom, implicit
constructibility, and<br>
non-nullity;<br>
- Merge the concept of "value companion" (`.val` type)
into the null-restricted<br>
refinement type of implicitly constructible value
classes;<br>
- Allow programmers to annotate type uses to explicitly
exclude or affirm nulls<br>
in the value set;<br>
- Provide some degree of runtime nullness checking to
detect null pollution;<br>
- Annotating an existing API (one based on identity
classes) with additional<br>
nullness information should be binary- and
source-compatible.<br>
<br>
The last goal is a source of strong constraints, and not
one to be taken<br>
lightly. If an existing API that specifies "this method
never returns null"<br>
cannot be compatibly migrated to one where this
constraint is reflected in the<br>
method declaration proper, the usefulness of
null-exclusion types is greatly<br>
reduced; library maintainers will be put to a bad choice
of forgoing a feature<br>
that will make their APIs safer, or making an
incompatible change in order to do<br>
so. If we were building a new language from scratch,
the considerations might<br>
be different, but we do not have that luxury. "Just
copying" what other<br>
languages have done here is a non-starter. <br>
<br>
### Interoperation between nullable and non-nullable
types<br>
<br>
We enable conversions between a nullable type and a
compatible null-excluding<br>
refinement type by adding new widening and narrowing
conversions between `T?`<br>
and `T!` that have analogous semantics to the existing
boxing and unboxing<br>
conversions between `Integer` and `int`. Just as with
boxing and unboxing,<br>
widening from a non-nullable type to a nullable type is
unconditional and never<br>
fails, and narrowing from a nullable type to a
non-nullable type may fail by<br>
throwing `NullPointerException`. These conversions for
null-excluding types<br>
would be sensible in assignment context, cast context,
and method invocation<br>
context (both loose and strict, unlike boxing for
primitives today.) This would<br>
allow existing assignments, invocation, and overload
applicability checks to<br>
continue to work even after migrating one of the types
involved, as required for<br>
source-compatibility.<br>
<br>
Checking for bad values can mirror the approach taken
for generics. When a<br>
richer compile-time type system erases to a less-rich
runtime type system, type<br>
safety derives from a mix of compile-time type checking
and synthetic runtime<br>
checks. In both cases, there is a possibility of
pollution which can be<br>
injected at the boundary between legacy and new code, by
malicious code, or<br>
through injudicious use of unchecked casts and raw
types. And like generics, we<br>
would like to offer the possibility that if a program
compiles in its entirety<br>
with no unchecked warnings, null-excluding types will
not be observed to contain<br>
null. To achieve this, we will need a combination of
runtime checks, new<br>
unchecked warnings, and possibly restrictions on
initialization. <br>
<br>
The intrusion on the type-checking of generics here is
considerable; nullity<br>
will have to be handled in type inference, bounds
conformance, subtyping, etc.<br>
In addition, there are new sources of heap pollution and
new conditions under<br>
which a varaible may be polluted. The _Universal
Generics_ JEP outlines a<br>
number of unchecked warnings that must be issued in
order to avoid null<br>
pollution in type variables that might be instantiated
either with a nullable or<br>
null-excluding type. While this work was designed for
`ref` and `val` types,<br>
much of it applies directly to null-excluding types.<br>
<br>
The liberal use of conversion rather than subtyping here
may be surprising to<br>
readers who are familiar with other languages that
support null-excluding types.<br>
At first, it may appear to be "giving up all the
benefit" of having annotated<br>
APIs for nullness, since a nullable value may be
assigned directly to a<br>
non-nullable type without requiring a cast. But the
reality is that for the<br>
first decade at least, we will at best be living in a
mixed world where some<br>
APIs are migrated to use nullness information and some
will not, and forcing<br>
users to modify code that uses these libraries (and then
do so again and again<br>
as more libraries migrate) would be an unnacceptable tax
on Java users, and a<br>
deterrent to libraries migrating to use these features.
<br>
<br>
Starting from `T! <: T?` -- and forcing explicit
conversions when you want to go<br>
from nullable to non-nullable values -- does seem an
obvious choice if you have<br>
the luxury of building a type system from scratch. But
if we want to make<br>
migration to null-excluding types a source-compatible
change for libraries and<br>
clients, we cannot accept a strict subtyping approach.
(Even if we did, we<br>
could still only use subtyping in one direction, and
would have to add an<br>
additional implicit conversion for the other direction
-- a conversion that is<br>
similar to the narrowing conversion proposed here.)<br>
<br>
Further, primitives _already_ use boxing and unboxing
conversions to go between<br>
their nullable (box) and non-nullable (primitive)
forms. So choosing subtyping<br>
for references (plus an unbalanced implicit conversion)
and boxing/unboxing<br>
conversion for primitives means our treatment of
null-excluding types is<br>
gratuitously different for primitives than for other
classes.<br>
<br>
Another consequence of wanting migration compatibility
for annotating a library<br>
with nullness constraints is that nullness constraints
cannot affect overload<br>
selection. Compatibility is not just for clients, it is
also for subclasses.<br>
<br>
### Null exclusion for implicitly constructible value
classes<br>
<br>
Implicitly constructible value classes go particularly
well with null exclusion,<br>
because we can choose a memory representation that
_cannot_ encode null,<br>
enabling a more compact and direct representation. <br>
<br>
The Valhalla JVM has support for such a representation,
and so we describe the<br>
null-exclusion type of an implicitly constructible value
class as _strongly null<br>
excluding_. This means that its null exclusion is
reified by the JVM. Such a<br>
variable can never be seen to contain null, because null
simply does not have a<br>
runtime representation for these types. This is only
possible because these<br>
classes are implicitly constructible; that the default
zero value written by the<br>
JVM is known to be a valid value of the domain. As with
primitives, these types<br>
are explicitly safe to use uninitialized. <br>
<br>
A strongly null-excluding type will have a type mirror,
as type mirrors describe<br>
reifiable types. <br>
<br>
### Null exclusion for other classes<br>
<br>
For identity classes and non-implicitly-constructible
value classes, the story<br>
is not quite as nice. Since there is no JVM
representation of "non-nullable<br>
String", the best we can do is translate `String!` to
`String` (a form of<br>
erasure), and then try to keep the nulls at bay. This
means that we do not get<br>
the flattening or density benefits, and null-excluding
variables may still be<br>
subject to heap pollution. We can try to minimize this
with a combination of<br>
static type checking and generated runtime checks. We
refer to the<br>
null-exclusion type of an identity or non-implicitly
constructible value class<br>
as _weakly null-excluding_.<br>
<br>
There is an additional source of potential null
pollution, aside from the<br>
sources analogous to generic heap pollution: the JVM
itself. The JVM<br>
initializes references in the heap to null. If
`String!` erases to an ordinary<br>
`String` reference, there is at least a small window in
time when this<br>
supposedly non-nullable field contains null. We can
erect barriers to reduce<br>
the window in which this can be observed, but these
barriers will not be<br>
foolproof. For example, the compiler could enforce that
a field of type<br>
`String!` either has an initializer or is definitely
assigned in every<br>
constructor. However, if the receiver escapes during
construction, all bets are<br>
off, just as they are with initialization safety for
final fields.<br>
<br>
We have a similar problem with arrays of `String!`;
newly created arrays<br>
initialize their elements to the default value for the
component type, which is<br>
`null`, and we don't even have the option of requiring
an initializer as we<br>
would with fields. (Since a `String![]` is also a
`String[]`, one option is to<br>
to outlaw the direct creation of arrays of weakly
null-excluding types, instead<br>
providing reflective API points which will safely create
the array and<br>
initialize all elements to a non-null value.)<br>
<br>
A weakly null-excluding type will not have a type
mirror, as the nullity<br>
information is erased for these types. Generic
signatures would be extended to<br>
represent null-exclusion, and similarly the `Type`
hiearchy would reflect such<br>
signatures. <br>
<br>
Because of erasure and the new possibilities for
pollution, allowing<br>
null-exclusion types for identity classes introduces
significant potential new<br>
complexity. For this reason, we may choose a staged
approach where<br>
null-restricted types are initially limited to the
strongly null-restricted<br>
ones.<br>
<br>
### Null exclusion for other value classes<br>
<br>
Value classes that are not implicitly constructible are
similar to identity<br>
classes in that their null-exclusion types are only
weakly null-excluding.<br>
These classes are the ones for which the author has
explicitly decided that the<br>
default zero value is not a valid member of the domain,
so we must ensure that<br>
in no case does this invalid value ever escape. This
effectively means that we<br>
must similarly erase these types to a nullable
representation to ensure that the<br>
zero value stays contained. (There are limited heroics
the VM can do with<br>
alternate representations for null when these classes
are small and have readily<br>
identifiable slack bits, but this is merely a potential
optimization for the<br>
future.) <br>
<br>
### Atomicity<br>
<br>
Primitives additionally have the property that
larger-than-32-bit primitives<br>
(`long` and `double`) may tear under race. The
allowance for tearing was an<br>
accomodation to the fact that numeric code is often
performance-critical, and so<br>
a tradeoff was made to allow for more performance at the
cost of less safety for<br>
incorrect programs. The corresponding box types, as
well as primitive variables<br>
declared `volatile`, are guaranteed not to tear, even
under race. (See the<br>
document entitled "Understanding non-atomicity and
tearing" for more detail.)<br>
<br>
Implicitly constructible value classes can be declared
as "non-atomic" to<br>
indicate that its null-exclusion type may tear under
race (if not declared<br>
`volatile`), just as with `long` and `double`. The
classes `Long` and `Double`<br>
would be declared non-atomic (though most
implementations still offer atomic<br>
access for 64-bit primitives.)<br>
<br>
### Flattening<br>
<br>
Flattening in the heap is an emergent property, which is
achieved when we give<br>
up the degrees of freedom that would prevent flattening:<br>
<br>
- Identity prevents flattening entirely;<br>
- Nullability prevents flattening in the absence of
heroics involving exotic<br>
representations for null; <br>
- The inability to use a class without initialization
requires nullability at<br>
the VM representation level, undermining flattening;<br>
- Atomicity prevents flattening for larger value
objects.<br>
<br>
Putting this together, the null-exclusion type of
implicitly constructible value<br>
classes is flattenable in the heap when the class is
non-atomic or the layout is<br>
suitably small. For ordinary value classes, we can
still get flattening in the<br>
calling convention: all identity-free types can be
flattened on the stack,<br>
regardless of layout size or nullability.<br>
<br>
### Summarizing null-exclusion<br>
<br>
The feature described so far is at the weak end of the
spectrum of features<br>
described by "non-nullable types". We make tradeoffs to
enable gradual<br>
migration compatibility, moving checks to the boundary
-- where in some cases<br>
they might not happen due to erasure, separate
compilation, or just dishonest<br>
clients. <br>
<br>
Users may choose to look at this as "glass X% full" or
"glass (100-X)% empty".<br>
We can now more clearly say what we mean, migrate
incrementally towards more<br>
explicit and safe code without forking the ecosystem,
and catch many errors<br>
earlier in time. On the other hand, it is less explicit
where we might<br>
experience runtime failures, because autoboxing makes
unboxing implicit. And<br>
some users will surely complain merely because this is
not what their favorite<br>
language does. But it is the null-exclusion we can
actually have, rather than<br>
the one we wish we might have in an alternate universe.
<br>
<br>
This approach yields a significant payoff for the
Valhalla story. Valhalla<br>
already had to deal with considerable new complexity to
handle the relationship<br>
between reference and value types -- but this new
complexity applied only to<br>
primitive classes. For less incremental complexity, we
can have a more uniform<br>
treatment of null-exclusion across all class types. The
story is significantly<br>
simpler and more unified than we had previously: <br>
<br>
- Everything, including the legacy primitives, is an
object (an instance of<br>
some class);<br>
- Every type, including the legacy primitives, is
derived from a class;<br>
- All types are reference types (they refer to
objects), but some reference<br>
types (non-nullable references to implicitly
constructible objects) exhibit<br>
the runtime behavior of primitives;<br>
- Some reference types exclude null, and some
null-excluding reference types<br>
are reifiable with a known-good non-null default;<br>
- Every type can have a corresponding null-exclusion
type.<br>
<br>
## Planning for a null-free future (?)<br>
<br>
Users prefer working with unnanotated types (e.g.,
`Foo`) rather than explicitly<br>
annotated types (`Foo!`, `Foo?`), where possible. The
unannotated type `Foo`<br>
could mean one of three things: an alias for `Foo!`, an
alias for `Foo?`, or a<br>
type of "raw" (unknown) nullity. Investigations into
null-excluding type<br>
systems have shown that the better default would be to
treat an unannotated name<br>
as indicating non-nullability, and use explicitly
nullable types (`T?`) to<br>
indicate the presence of null, because returning or
accepting null is generally<br>
a less common case. Of course, today `String` means
"possibly nullable String"<br>
in Java, meaning that, yet again, we seem to have chosen
the wrong default. <br>
<br>
Our friends in the `C#` community have explored the
possibility of a<br>
"flippening". `C#` started with the Java defaults, and
later provided a<br>
compiler mode to flip the default on a per-module basis,
with checking (or<br>
pollution risk) at the boundary between modules with
opposite defaults. This is<br>
an interesting experiment and we look forward to seeing
how this plays out in<br>
the `C#` ecosystem. <br>
<br>
Alternately, another possible approach for Java is to
continue to treat the<br>
unadorned name as having "raw" or "unknown" nullity,
encouraging users to<br>
annotate types with either `!` or `?`. This approach
has been partially<br>
explored in the `JSpecify` project. Within this
approach is a range of options<br>
for what the language will do with such types; there is
a risk of flooding users<br>
with warnings. We may want to leave such analysis to
extralinguistic type<br>
checkers, at least initially -- but we would like to not
foreclose on the<br>
possibility of an eventual flippening.<br>
<br>
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