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<font size="4"><font face="monospace">Just a reminder -- this topic
is out of scope for valhalla-dev, which is for implementation
discussions, not language design. <br>
<br>
Let's just say that we are more than aware that there is overlap
between nullity and value types, and that some developers
really^3 like the idea of having nullity control in the
language. We can leave it at that for now. <br>
<br>
(BTW this has been explored not only by c#, but by several
explorations in the Java ecosystem, including the checkers
framework and the jspecify effort. (Which are also out of scope
for this list.))<br>
</font></font><br>
<div class="moz-cite-prefix">On 1/19/2023 3:19 PM, Red IO wrote:<br>
</div>
<blockquote type="cite" cite="mid:CABKyW1t6+47Mbf8YBHfUtKC-KOwB0hAV6DvBba-RGsGtx=+aYg@mail.gmail.com">
<div dir="auto">If we talk about the user model for the explicit
nullability of refrences we should think about allowing the
developer to turn it around. Meaning that every reference is
treated non null when not marked otherwise. But this should
definitely strictly be "an option" that you can enable somehow.
<div dir="auto">This can be really beneficial since the compiler
can mark every point where an (default) non nullable is
assigned (potential) null value or a nullable variable is
dereferenced at all. </div>
<div dir="auto">I just got the idea of inverting the nullability
default from c# which has this feature. I recently flipped it
on one of my projects and the entire code lit up with
warnings. Showing countless spots where a null check was
missing.</div>
<div dir="auto">It would be logical to use "!" for explicit
non-null and "?" for explicit nullable. With 1 being optional
in each case rather the default is inversed or not.</div>
<div dir="auto"><br>
</div>
<div dir="auto">Great regards </div>
<div dir="auto">RedIODev </div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Wed, Jan 11, 2023, 18:30
Anderson Vasconcelos Pires <<a href="mailto:andvasp@gmail.com" moz-do-not-send="true" class="moz-txt-link-freetext">andvasp@gmail.com</a>>
wrote:<br>
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<div dir="ltr">
<div dir="ltr">Thanks for the prompt response!<br>
</div>
<div class="gmail_quote"><br>
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<div dir="ltr"><span style="font-family:arial,helvetica,sans-serif;font-size:12pt">Mostly
prototyping a proposal to remove Q-types
(descriptor that starts with Q...;) and
replace them by an attribute on methods and
fields indicating if the value can be null.</span><br>
</div>
</blockquote>
<div>see <a href="https://mail.openjdk.org/pipermail/valhalla-spec-experts/2023-January/002217.html" target="_blank" rel="noreferrer" moz-do-not-send="true" class="moz-txt-link-freetext">https://mail.openjdk.org/pipermail/valhalla-spec-experts/2023-January/002217.html</a></div>
<div><span style="color:rgb(34,34,34);font-family:Arial,Helvetica,sans-serif;font-size:small"></span></div>
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<div><br>
</div>
<div>So if you want to remove Q types it means that L type
does not need the identity information, right? </div>
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<div dir="ltr">
<div><br>
</div>
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</blockquote>
<div>There is no notion of .val or .ref, if you want a
field to be flattened, use a @Zero-Default and
declare the field @NonNull.<br>
</div>
</div>
</blockquote>
<div><br>
</div>
<div>Pretty good if we do not need to use .val or .ref!</div>
<div>
<div><br>
Could the @NonNull be replaced by "!" but writing the
attribute in the class file?<br>
</div>
<div><br>
</div>
<div>I Like @NonNull but I believe the "!" would be more
practical.</div>
<div> <br>
</div>
<div>As @Nullable is the default, maybe it would not be
necessary to have it.</div>
</div>
<div> </div>
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<div>
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<div dir="ltr"><span style="font-family:arial,helvetica,sans-serif;font-size:12pt">yes,
the prototype does not do any verification
that the compiler does, i'm too lazy for
that but the VM will catch you and throw an
Error</span></div>
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<div><br>
</div>
<div>Pretty good for a prototype! </div>
<div> </div>
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<div>
<div>About the mutability of fields,
fundamentally, value type is about saying, i
give up the notion of identity, if you have no
identity, you have no location in memory (you
can have more than one if you prefer) so the VM
is free to pass by reference or pass by value.
So you can not mutate a field, because you can
only do that if you have one location in memory.<br>
</div>
<div><br>
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<div><br>
</div>
<div>Ok for mutability but now I am a little bit confused
about references, identity and memory location. Value
class is a reference type that does not have identity.
So it means that it can be allocated in the heap, right?
So allocating it in the heap will have a memory
location, do not? </div>
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<div dir="ltr"><span style="font-family:arial,helvetica,sans-serif;font-size:12pt">regards,</span><br>
</div>
</blockquote>
<div>Rémi<br>
</div>
<div><br>
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<div><br>
</div>
<div>Thanks for the explanation!</div>
<div><br>
</div>
<div>Regards,</div>
<div>Anderson. </div>
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<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Tue, Jan 10,
2023 at 7:39 PM Remi Forax <<a href="mailto:forax@univ-mlv.fr" target="_blank" rel="noreferrer" moz-do-not-send="true" class="moz-txt-link-freetext">forax@univ-mlv.fr</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px
0px 0px 0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">Hi all,<br>
i've developed a small prototype of what the
semantics for the next round could be<br>
<a href="https://github.com/forax/civilizer/" rel="noreferrer noreferrer" target="_blank" moz-do-not-send="true" class="moz-txt-link-freetext">https://github.com/forax/civilizer/</a><br>
<br>
The prototype works with the latest valhalla
early access build<br>
<a href="https://jdk.java.net/valhalla/" rel="noreferrer noreferrer" target="_blank" moz-do-not-send="true" class="moz-txt-link-freetext">https://jdk.java.net/valhalla/</a><br>
<br>
Basically, everything is an object, using @Value
in front of a class or a record transforms it to
a value class, a class with no identity<br>
(== compares the fields, synchronized throws an
IllegalMonitorStateException, new
WeakRef<>(...) throws an
IdentityException).<br>
Adding @ZeroDefault means that the default value
of the value class which is not null is all
fields filled with zero.<br>
<br>
@NonNull allows to declare a non-null type
(there is also @Nullable which is the default if
there is no null aware annotation).<br>
For parameters, like Kotlin, sending null to a
parameter of a method annotated with @NonNull
throws a NPE.<br>
For a field a zero-default annotated with
@NonNull ask for flattening, trying to store
null also throws a NPE.<br>
For a field which is not a zero-default, even if
declared @NonNull, the runtime will *not* throw
a NPE because the field is null before reaching
its initialization assignment in the constructor
(and a constructor can call any methods).<br>
<br>
From the VM POV, a Q-type never appears inside a
method descriptor. It appears inside field
descriptor but putfield, getfield and withfield
allows a field declared as a Q-type to be
accessed as a L-type (this is currently
simulated for getfield and putfield with an
invokedynamic). This is very similar to the way
the VM deals with arrays. And the VM knows if a
parameter is a Q-type or not despite the fact
that only L-type appears in the method
descriptor because the Q-type are declared in
the Preload table of the class. <br>
<br>
It means that if a class/record is not accessed
directly by a constructor but by some static
factories (like Optional) then moving in between
an identity type, a value type (with
zero-default or not) are backward compatible
changes.<br>
<br>
regards,<br>
Rémi<br>
</blockquote>
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