Into

[Brian Goetz]

Right.  You want to do the upstream stuff in parallel, then you want to 
do the downstream stuff (a) serially, (b) in the current thread, and 
probably (c) in encounter order.

So, assume for sake of discussion that we have some form of .toList(), 
whether as a "native" operation or some sort of reduce/combine/tabulate. 
  Then you can say:

   parallelStream()...toList().forEach(...)

and the list-building will happen in parallel and then forEach can help 
sequentially.

Given that, is there any reason left for sequential()?



On 12/22/2012 11:55 AM, Joe Bowbeer wrote:
> The main use case is sequential().forrEach(), which inserts any ol'
> for-loop into a computation.
>
> On Dec 21, 2012 1:48 PM, "Brian Goetz" <brian.goetz at oracle.com
> <mailto:brian.goetz at oracle.com>> wrote:
>
>     If we get rid of into(), and replace it with an explicit reduction
>     [1], then we may be able to get rid of sequential() too.
>
>     The primary use case for sequential() was for using non-thread-safe
>     Collections as an into() target.  The convoluted into() turns the
>     problem around on the target, calling target.addAll(stream) on it,
>     and the typical implementation of into() is like the one in Collection:
>
>          default void addAll(Stream<? extends E> stream) {
>              if (stream.isParallel())
>                  stream = stream.sequential();
>              stream.forEach(this::add);
>          }
>
>     or more compactly
>
>          default void addAll(Stream<? extends E> stream) {
>              stream.sequential().forEach(__this::add);
>          }
>
>     since sequential() is now a no-op on sequential streams.
>
>     The code looks pretty, but the implementation is not; sequential()
>     is a barrier, meaning you have to stop and collect all the elements
>     into a temporary tree, and then dump them into the target.  But it
>     is not obvious that it is a barrier, so people will be surprised.
>       (And on infinite streams, it's bad.)
>
>     What used to be implicit in sequential() can now be made explicit with:
>
>        if (stream.isParallel())
>           stream = stream...__whateverWeCallOrderPreservingI__nto().stream()
>
>     That offers similar semantics and at least as good performance,
>     while also being more transparent and requiring one fewer weird
>     stream operation.
>
>     (I don't yet see the way to getting rid of .parallel(), but we can
>     possibly move it out of Stream and into a static method
>     Streams.parallel(stream), at some loss of discoverability.  But we
>     can discuss.)
>
>
>     [1] Actually, we have to replace it with two explicit reductions, or
>     more precisely, a reduction and a for-eaching.  One is the pure
>     reduction case that involves merging, and is suitable for
>     non-thread-safe collections (and required if order preservation is
>     desired); the other is the concurrent case, where we bombard a
>     concurrent collection with puts and hope it manages to sort them
>     out. The two are semantically very different; one is a reduce and
>     the other is a forEach, and so they should have different
>     manifestations in the code.  Though there are really no concurrent
>     Collections right now anyway (though you could fake a concurrent Set
>     with a concurrent Map.)
>
>
>
>     On 12/21/2012 12:50 PM, Brian Goetz wrote:
>
>         I'm starting to dislike "into".
>
>         First, it's the only stream method which retains mutable state
>         from the
>         user.  That's not great.
>
>         Second, the parallel story is bad.  People are going to write
>
>             list.parallel(e -> e+1).into(new ArrayList<>());
>
>         which will do a whole lot of trivial computation in parallel,
>         wait on
>         the barrier implicit in sequential(), and then do an O(n) serial
>         thing.
>
>         Third, the semantics are weird; we do this clever trick where
>         collections have to decide whether to do insertion in serial or
>         parallel.  But as we all learned from Spinal Tap, there's a fine
>         line
>         between clever and stupid.
>
>         Instead, we could treat this like a mutable reduce, where leaves are
>         reduced to a List, and lists are merged as we go up the tree.
>           Even with
>         dumb merging is still going to be much faster than what we've
>         got now;
>         no barrier, no buffer the whole thing and copy, and the worst serial
>         step is O(n/2) instead of O(n).  So probably 3x better just by
>         improving
>         the serial fractions.  But with a smarter combination step, we
>         can do
>         better still.  If we have a "concatenated list view" operation (List
>         concat(List a, List b)), which returns a read-only, conc-tree
>         representation), then the big serial stage goes away.
>
>         And, of course, building atop reduce makes the whole thing simpler;
>         there are fewer ops that have their own distinct semantics, and the
>         semantics of into() is about as weird as you get.
>
>
>         Now that the tabulators framework gets users comfortable with the
>         explicit choice between functional and concurrent aggregation for
>         tabulation, it is a much shorter hop to get there.  So let's
>         build on
>         that and find some sort of way to surface mutable and concurrent
>         versions of "into".  (Currently we have no good concurrent
>         list-shaped
>         collections, but hopefully that changes.)
>
>         Something like:
>
>             Stream.tabulate(collector(__ArrayList::new))
>             Stream.tabulate(__concurrentCollector(__ConcurrentFooList::new))
>
>         Maybe with some rename of tabulate.
>
>         I think there's a small reorganization of naming lurking here
>         (involving
>         tabulate, Tabulator, ConcurrentTabulator, MutableReducer,
>         reduce) that
>         recasts into() either as an explicit functional or concurrent
>         tabulation.  And one more tricky+slow special-purpose op bites
>         the dust,
>         in favor of something that builds on our two favorite
>         primitives, fold
>         (order-preserving) and forEach (not order-preserving.)
>
>