flatMap (was: Updated Java 8 Stream examples in GS Collections Kata) (original) (raw)

Brian Goetz brian.goetz at oracle.com
Tue Nov 13 07:42:54 PST 2012

The things I found awkward using in the kata were flatMap

This is a complaint we received over and over again in the "Hack Day" sessions -- it is pretty clear we are not there yet on flatMap.

  1. It is not obvious flatMap is the best name, as it sets expectations for Scala users that will not be met. Perhaps mapMulti? explode?

  2. The API form is definitely not there yet. But, the "obvious" suggestion is clearly wrong. Assume for sake of argument that there will be only one version of flatMap. What everyone thinks it should look like (after a few seconds of thought) is:

Stream flatMap(Mapper<T, Collection>)

But, this is awful (except when you already happen to have a Collection lying around, which will be sometimes but by no means all of the time.) This forces (a) the client to create and populate a Collection in the lambda (yuck!), usually an extra collection to be created even when the element maps to nothing, and then forces an iteration (with Iterator) on the other side. So, in the case where there's no collection lying around, the client code is larger, uglier, and in any case the whole thing is significantly slower. The current version has a very nice property: when the element maps to nothing, there's no work to do.

This is further challenging because of erasure. If we had:

Stream flatMap(Mapper<T, Collection>)

we might also want

Stream flatMap(Mapper<T, T[]>) or Stream flatMap(Mapper<T, Stream>) or Stream flatMap(Mapper<T, Streamable>)

But, we can't overload these since their erasure is all "Mapper".

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