+1 I've seen this mistake myself in some PRs.
On Thu, 8 Dec 2016 at 06:10 Ben Chambers <bchamb...@google.com.invalid> wrote: > +1 -- This seems like the best option. It's a mechanical change, and the > compiler will let users know it needs to be made. It will make the mistake > much less common, and when it occurs it will be much clearer what is wrong. > > It would be great if we could make the mis-use a compiler problem or a > pipeline construction time error without changing the names, but both of > these options are not practical. We can't hide the expansion method, since > it is what PTransform implementations need to override. We can't make this > a construction time exception since it would require adding code to every > PTransform implementation. > > On Wed, Dec 7, 2016 at 1:55 PM Thomas Groh <tg...@google.com.invalid> > wrote: > > > +1; This is probably the best way to make sure users don't reverse the > > polarity of the PCollection flow. > > > > This also brings PInput.expand(), POutput.expand(), and > > PTransform.expand(PInput) into line - namely, for some composite thing, > > "represent yourself as some collection of primitives" (potentially > > recursively). > > > > On Wed, Dec 7, 2016 at 1:37 PM, Kenneth Knowles <k...@google.com.invalid> > > wrote: > > > > > Hi all, > > > > > > I want to bring up another major backwards-incompatible change before > it > > is > > > too late, to resolve [BEAM-438]. > > > > > > Summary: Leave PInput.apply the same but rename PTransform.apply to > > > PTransform.expand. I have opened [PR #1538] just for reference (it took > > 30 > > > seconds using IDE automated refactor) > > > > > > This change affects *PTransform authors* but does *not* affect pipeline > > > authors. > > > > > > This issue was filed a long time ago. It has been a problem many times > > with > > > actual users since before Beam started incubating. This is what goes > > wrong > > > (often): > > > > > > PCollection<Foo> input = ... > > > PTransform<PCollection<Foo>, ...> transform = ... > > > > > > transform.apply(input) > > > > > > This type checks and even looks perfectly normal. Do you see the error? > > > > > > ... what we need the user to write is: > > > > > > input.apply(transform) > > > > > > What a confusing difference! After all, the first one type-checks and > the > > > first one is how you apply a Function or Predicate or > > SerializableFunction, > > > etc. But it is broken. With transform.apply(input) the transform is not > > > registered with the pipeline at all. > > > > > > We obviously can't (and don't want to) change the most core way that > > > pipeline authors use Beam, so PInput.apply (aka PCollection.apply) must > > > remain the same. But we do need a way to make it impossible to mix > these > > > up. > > > > > > The simplest way I can think of is to choose a new name for the other > > > method involved. Users probably won't write transform.expand(input) > since > > > they will never have seen it in any examples, etc. This will just make > > > PTransform authors need to do a global rename, and the type system will > > > direct them to all cases so there is no silent failure possible. > > > > > > What do you think? > > > > > > Kenn > > > > > > [BEAM-438] https://issues.apache.org/jira/browse/BEAM-438 > > > [PR #1538] https://github.com/apache/incubator-beam/pull/1538 > > > > > > p.s. there is a really amusing and confusing call chain: > > PCollection.apply > > > -> Pipeline.applyTransform -> Pipeline.applyInternal -> > > > PipelineRunner.apply -> PTransform.apply > > > > > > After this change and work to get the runner out of the loop, it > becomes > > > PCollection.apply -> Pipeline.applyTransform -> PTransform.expand > > > > > >
