RFR: 8351565: Implement JEP 502: Stable Values (Preview) [v40]

[Per Minborg]

On Thu, 3 Apr 2025 14:46:49 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Per Minborg has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Make the sqrt example different
>
> src/java.base/share/classes/java/lang/StableValue.java line 211:
> 
>> 209:  *             StableValue.function(CACHED_KEYS, LOG2_ORIGINAL);
>> 210:  *
>> 211:  *     public static double log2(int a) {
> 
> IMHO this example reveals a lack of compositionality of the partial functions we have. If the functions returned optional (to indicate "not defined"), then code like this would become much simpler to express:
> 
> 
> return LOG2_CACHED.apply(a)
>            .orElse(LOG2_ORIGINAL.apply(a));
> 
> 
> It's not just that this is more fluent -- this is also using the caching function to check whether the input is cached or not -- instead of having a separate check (potentially duplicating the costs).

I agree with this observation. We should look at providing true partial functions using for example `Optional`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23972#discussion_r2028394873