RFR: 8319123: Implementation of JEP-461: Stream Gatherers (Preview) [v7]

[Viktor Klang]

On Tue, 14 Nov 2023 09:48:13 GMT, Rémi Forax <forax at openjdk.org> wrote:

> Hello, the relation between a stateless gatherer (default initializer) and the default combiner are not obvious to me.
> 
> A default initializer means stateless and a default combiner means sequential, so if i want the integrator of my stateless gatherer to be called in parallel, i need to not use the default combiner but at the same time the combiner I will pass as parameter will not be called because the gatherer is stateless ?
> 
> Having to create a combiner that will be not called seems weird. I'm sure i'm missing something ?

It stems from the observation that the nature of something being stateless does not mean that it doesn't have restrictions on encounter order (see forEach and forEachOrdered as an example). Parallelization means foregoing encounter-order (of execution -- not necessarily of output), so by requiring to specify a combiner to opt into parallel (unordered execution) means that the default is correct and safe under all evaluation modes, and opting into out-of-order execution is inspectable (in terms of ocular inspection of code).

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PR Comment: https://git.openjdk.org/jdk/pull/16420#issuecomment-1809940566