Factory methods for SequencedSet and SequencedMap

[forax at univ-mlv.fr]

> From: "Rafael Winterhalter" <rafael.wth at gmail.com>
> To: "Remi Forax" <forax at univ-mlv.fr>
> Cc: "core-libs-dev" <core-libs-dev at openjdk.org>, "joe darcy"
> <joe.darcy at oracle.com>
> Sent: Tuesday, January 21, 2025 10:17:35 AM
> Subject: Re: Factory methods for SequencedSet and SequencedMap

> Wouldn't this already be possible with today's union types?

> static <E, S extends Set<E> & SequencedSet<E>> S of(E... elements) { ... }

No, because when you have a type variable declared on a method, it means \forall, 
so here, the method of() has to work for all implementations of S, so all implementations of a SeqenceSet. 

To see the issue, you can fix the type argument, by example 
TreeSet<String> set = Set.<String, TreeSet<String>>of("foo") 

is obviously wrong but allowed by the declaration. 

> Then again, I do not think that the regular Set and Map implementations should
> be sequenced, mainly to avoid that tests suffer from this sequencing.

I do not disagree :) 

Rémi 

> Am So., 19. Jan. 2025 um 16:18 Uhr schrieb Remi Forax < [
> mailto:forax at univ-mlv.fr | forax at univ-mlv.fr ] >:

>> What can be done is to have Set.of()/Map.of() to delegate to
>> SequenceSet.of()/SequenceMap.of() so there is only one implementation at
>> runtime.

>> Also, technically, there is a way to change the return type in a binary
>> compatible way ... if union types are supported in the language.
>> In that case, Set.of() can be declared like this

>> static <E> Set<E> | SequenceSet<E> of(E... ) {
>> ...
>> }

>> because of erasure, the binary descriptor will use Set, but the Signature will
>> be Set<E> | SequenceSet<E> so the compiler will see the return type as a
>> SequencedSet .

>> Obviously, union types is a far bigger features than adding
>> SequenceSet/SequenceMap so it's a big if, but it may happen in the future.

>> Rémi

>>> From: "joe darcy" < [ mailto:joe.darcy at oracle.com | joe.darcy at oracle.com ] >
>>> To: "core-libs-dev" < [ mailto:core-libs-dev at openjdk.org |
>>> core-libs-dev at openjdk.org ] >
>>> Sent: Friday, January 17, 2025 6:30:40 PM
>>> Subject: Re: Factory methods for SequencedSet and SequencedMap

>>> On 1/16/2025 11:26 PM, Rafael Winterhalter wrote:

>>>> Would it even be possible to change the return types of Set.of(...) and
>>>> Map.of(...) without breaking binary compatibility?

>>> In short, no.
>> [...]

>>> -Joe
>> Rémi
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