Into (original) (raw)

Doug Lea dl at cs.oswego.edu
Sun Dec 23 04:51:01 PST 2012

On 12/22/12 17:15, Brian Goetz wrote:

(One thing I don't love is that they bring List/Map back into the API -- after all the work we did in groupBy to move it out. But this is more superficial than what groupBy/reduceBy did, so I'm probably OK with this -- especially if they are just sugar.)

My (long-standing :-) concern is the potential bug-tail of methods that introduce one more level of indirection on the already intractable binary-method problem. Method into() is asked to automagically resolve any mismatches between properties of the source and of the destination; where the properties mainly include ordered and parallel, but also keyed, sorted, indexed, random-access. The Collections framework is not especially good at helping you out here. One possibility for reducing complexity is to limit the number of cases, via toList, toSet, etc and allow the implementation to rely on particular properties of the internally chosen destination classes. This is not very exciting, but it also does not rule out doing something more ambitious later.

Another strategy (a variant of one discussed before) for collapsing one side of the binary method problem would be to somehow require that concrete Collection/Map classes support a stream-based constructor. This encounters a couple of snags though: (1) We can't require or default-define constructors, so would have to live with say void buildFromStream(Stream x) (default: if !empty die else sequential-forEach-add). (2) Usages lose the fluency look-and-feel, which bothers some people a lot: Usages must declare and use the destination in a separate statement than the pipleline expression.

I'd probably prefer this approach, in part because it does fully remove collections from the stream framework (by adding stream support to the collections framework, which we must do anyway.). But I realize that the fluent-API expectations for streams make it an unpopular choice.

Footnote: Recall that Scala deals with the binary-method-problem aspects by encoding some of these constraints in the type system and resolving a match. Which is sorta cool but still produces anomalies.

-Doug

More information about the lambda-libs-spec-experts mailing list