Characterizing stream operation

Remi Forax forax at
Fri Feb 15 15:46:08 PST 2013

On 02/15/2013 09:04 PM, Brian Goetz wrote:
> We've divided stream operations as follows:
> Intermediate operations.  Always lazy.  Always produce another stream.
> Stateful operations.  A kind of intermediate operation.  Currently 
> always transforms to the same stream type (e.g., Stream<T> to 
> Stream<T>), though this could conceivably change (we haven't found 
> any, though).  Must provide their own parallel implementation.  
> Parallel pipelines containing stateful operations are implicitly 
> "sliced" into segments on stateful operation boundaries, and executed 
> in segments.
> Terminal operations.  The only thing that kicks off stream 
> computation.  Produces a non-stream result (value or side-effects.)
> For each of these, once you perform an operation on a stream 
> (intermediate or terminal), the stream is *consumed* and no more 
> operations can be performed on that stream.  (Not entirely true, as 
> the TCK team will almost certainly point out to us eventually; there 
> are some ops that are no-ops and probably will succeed unless we add 
> consumed checks.)
> These names are fine from the perspective of the implementation; when 
> implementing an operation, you will be implementing one of these three 
> types, and conveniently there is a base type for each to subclass.
> From the user perspective, though, they may not be as helpful as some 
> alternative taxonomies, such as:
>  - lazy operation -- what we now call intermediate operation
>  - stateful lazy operation -- what we now call stateful
>  - consuming operation -- what we now call terminal
> These are good in that they keep a key characteristic -- when the 
> computation happens -- in full view.  However, they also create less 
> clean boundaries.  For example, iterator() is a consuming operation 
> from the perspective of the stream, but from the perspective of the 
> user, may be thought of as lazy.
> Thoughts on how to adjust this naming to be more intuitive to users?

lazy and terminal are Ok for me, stateful can be renamed to intermediate 


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