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

[Sam Pullara]

Honestly I would have expected only a single shape for flatMap:

>  Stream<V> flatMap(Mapper<T, Stream<V>>)


And it is used to replace doing map from T to Stream<V> resulting Stream<Stream<V>> and then flattening the result. Basically it should just return a concatenation of the resulting streams. Here is the definition in Scala's TraversableLike:

def flatMap[B](f: (A) ⇒ GenTraversableOnce[B]): Traversable[B]
[use case]
Builds a new collection by applying a function to all elements of this traversable collection and using the elements of the resulting collections.

Similarly for Optional you'd have:

Optional<V> flatMap(Mapper<T, Optional<V>>)

Avoiding the awkward Optional<Optional<V>> when you use normal map.

Sam

On Nov 13, 2012, at 7:42 AM, Brian Goetz <brian.goetz at oracle.com> wrote:

>> 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<T> flatMap(Mapper<T, Collection<T>>)
> 
> 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<T> flatMap(Mapper<T, Collection<T>>)
> 
> we might also want
> 
>  Stream<T> flatMap(Mapper<T, T[]>)
> or
>  Stream<T> flatMap(Mapper<T, Stream<T>>)
> or
>  Stream<T> flatMap(Mapper<T, Streamable<T>>)
> 
> But, we can't overload these since their erasure is all "Mapper".
> 
>