Civilizer - let's civilize those primitive types !

[Anderson Vasconcelos Pires]

Thanks for the prompt response!

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.
>
> see
> https://mail.openjdk.org/pipermail/valhalla-spec-experts/2023-January/002217.html
>

So if you want to remove Q types it means that L type does not need the
identity information, right?

>
> 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.
>

Pretty good if we do not need to use .val or .ref!

Could the @NonNull be replaced by "!" but writing the attribute in the
class file?

I Like @NonNull but I believe the "!" would be more practical.

As @Nullable is the default, maybe it would not be necessary to have it.


> 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
>
>
Pretty good for a prototype!


> 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.
>
>
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?

> regards,
>
> Rémi
>
>
Thanks for the explanation!

Regards,
Anderson.

>
> On Tue, Jan 10, 2023 at 7:39 PM Remi Forax <forax at univ-mlv.fr> wrote:
>
>> Hi all,
>> i've developed a small prototype of what the semantics for the next round
>> could be
>>   https://github.com/forax/civilizer/
>>
>> The prototype works with the latest valhalla early access build
>>   https://jdk.java.net/valhalla/
>>
>> 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
>> (== compares the fields, synchronized throws an
>> IllegalMonitorStateException, new WeakRef<>(...) throws an
>> IdentityException).
>> Adding @ZeroDefault means that the default value of the value class which
>> is not null is all fields filled with zero.
>>
>> @NonNull allows to declare a non-null type (there is also @Nullable which
>> is the default if there is no null aware annotation).
>> For parameters, like Kotlin, sending null to a parameter of a method
>> annotated with @NonNull throws a NPE.
>> For a field a zero-default annotated with @NonNull ask for flattening,
>> trying to store null also throws a NPE.
>> 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).
>>
>> 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.
>>
>> 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.
>>
>> regards,
>> Rémi
>>
>
>
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