partition

In std/seq.

partition fn partition<S : Seq<T>, K>(self: S, by: T -> K) : S
partition fn partition<K>(self: Stream<T>, by: T -> K) : Stream<T>

The values of self redistributed by the key by gives: each value once, at its own type, those of one key in one logical partition. The step is a stage boundary. The local backend has one partition per key on one worker and passes the values on in their order.