Parallel Collections - Creating Custom Parallel Collections - Scala Documentation

Just as it is possible to define custom sequential collections without defining their builders, it is possible to define parallel collections without defining their combiners. The consequence of not having a combiner is that transformer methods (e.g. map, flatMap, collect, filter, …) will by default return a standard collection type which is nearest in the hierarchy. For example, ranges do not have builders, so mapping elements of a range creates a vector.

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