flatMap

[Brian Goetz]

That's the obvious answer...like I said, it's just not the right answer 
here.  (It can be a convenience layer built atop of a more flexible 
primitive, though.)  Try writing something like "break a string into 
words" using the flatMap(Mapper<T,Stream<T>>), or more generally 
anything that naturally yields up elements one at a time.)  Also the 
Mapper<T,Stream<T>> is going to be a lot less efficient.  Like I said, 
it can be part of the answer, but it is absolutely the wrong primitive.

Java isn't Scala.  What works in Scala libraries may well not work in 
Java libraries, because the language is different.  So the "right" 
answer in one language is not necessarily the right answer in another. 
(Related example: if you have tuples, a method like 
"partition(Predicate)" is more natural than if you don't.  So the 
partition method is a no-brainer in Scala but entails far more tradeoffs 
in Java, and might be on the other side of the line for that reason.)



On 11/13/2012 12:33 PM, Sam Pullara wrote:
> Honestly I would have expected only a single shape for flatMap:
>
>>  Stream<V> flatMap(Mapper<T, Stream<V>>)
>
> Where the result is a concatenation of the Streams returned. Kind of
> like doing a map from T to Stream<V> ending up with a Stream<Stream<V>>
> and then calling flatten() on that Stream. Is this not the normal
> definition? Here is the definition from Scala:
>
>
>         defflatMap[B](f: (A) ⇒ GenTraversableOnce
>         <http://www.scala-lang.org/archives/downloads/distrib/files/nightly/docs/library/scala/collection/GenTraversableOnce.html>[B]):
>         Traversable
>         <http://www.scala-lang.org/archives/downloads/distrib/files/nightly/docs/library/scala/collection/Traversable.html>[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.
> For example:
>
> def  getWords(lines:Seq[String]):Seq[String] = lines flatMap (line=>  line split"\\W+  <smb://W+>")
>
> The type of the resulting collection is guided by the static type of
> traversable collection. This might cause unexpected results sometimes.
> For example:
>
> // lettersOf will return a Seq[Char] of likely repeated letters, instead of a Set
> def  lettersOf(words:Seq[String]) = words flatMap (word=>  word.toSet)
>
> // lettersOf will return a Set[Char], not a Seq
> def  lettersOf(words:Seq[String]) = words.toSet flatMap (word=>  word.toSeq)
>
> // xs will be a an Iterable[Int]
> val  xs =Map("a"  ->List(11,111),"b"  ->List(22,222)).flatMap(_._2)
>
> // ys will be a Map[Int, Int]
> val  ys =Map("a"  ->List(1  ->11,1  ->111),"b"  ->List(2  ->22,2  ->222)).flatMap(_._2)
>
> B
>     the element type of the returned collection.
> f
>     the function to apply to each element.
> returns
>     a new traversable collection resulting from applying the given
>     collection-valued function |f| to each element of this traversable
>     collection and concatenating the results.
>
> Definition Classes
>     TraversableLike
>     <http://www.scala-lang.org/archives/downloads/distrib/files/nightly/docs/library/scala/collection/TraversableLike.html> →
>     GenTraversableLike
>     <http://www.scala-lang.org/archives/downloads/distrib/files/nightly/docs/library/scala/collection/GenTraversableLike.html> →
>     FilterMonadic
>     <http://www.scala-lang.org/archives/downloads/distrib/files/nightly/docs/library/scala/collection/generic/FilterMonadic.html>
>
>
>
> Sam
>
> On Nov 13, 2012, at 7:42 AM, Brian Goetz <brian.goetz at oracle.com
> <mailto: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".
>>
>>
>