Mostly my thoughts are the same as Robert's. Use #3 whenever possible,
fallback to #1 otherwise, but please consider using informative names for
your methods in all cases.
#1 GBK.<T>create(): IMO this pattern is best only for transforms where
withBar is optional or there is no such method, as in GBK. If it is
mandatory, it should just be required in the first method, eliding the
issue, as in ParDo.of(DoFn<I, O>), MapElements.via(...), etc, like you say
in your concluding remark.
#2 FooBuilder<?> FooBuilder.create(): this too - if you are going to fix
the type, fix it first. If it is optional and Foo<?> is usable as a
transform, then sure. (it would have be something weird like Foo<InputT,
OutputT, ?> extends PTransform<InputT, OutputT>)
#3 Foo.create(Bar<T>): this is best. Do this whenever possible. From my
perspective, instead of "move the param to create(...)" I would describe
this as "delete create() then rename withBar to create". Just skip the
second step and you are in an even better design, withBar being the
starting point. Just like ParDo.of and MapElements.via.
#4 Dislike this, too, for the same reasons as #2 plus code bloat plus user
confusion.
Side note since you use this method in all your examples: This kind of use
of "create" is a bad method name. There may be no new object "created".
Sometimes we have no better idea, but create() is a poor default. For GBK
both are bad: create() (we really only need one instance so why create
anything?) and create(<boolean>) (what is the unlabeled boolean?). They
would be improved by GBK.standard() and GBK.fewKeys() or some such. I tend
to think that focusing on this fine polish eliminates a lot of cases for
the generalized question.
Kenn
On Thu, Oct 6, 2016 at 2:10 PM Eugene Kirpichov
<kirpic...@google.com.invalid> wrote:
> Quite a few transforms in the SDK are generic (i.e. have type parameters),
> e.g. ParDo, GroupByKey, Keys / WithKeys, many connectors (TextIO, KafkaIO,
> JdbcIO, MongoDbGridFSIO etc - both read and write). They use different
> styles of binding the type parameters to concrete types in caller code.
>
> I would like us to make a decision which of those styles to recommend for
> new transform and connectors writers. This question is coming up rather
> frequently, e.g. it came up in JdbcIO and MongoDbGridFSIO.
>
> For the purpose of this discussion, imagine a hypothetical builder class
> that looks like this:
>
> class Foo<T> {
> private Bar<T> bar;
> private int blah;
>
> Foo<T> withBlah(int blah);
> }
>
> So far I've seen several styles of binding the type argument in a withBar()
> method vs. a creation method:
>
> 1. Binding at the creation method: e.g.:
>
> class Foo<T> {
> ...
> public static <T> Foo<T> create();
> public FooBuilder<T> withBar(Bar<T> bar);
> }
>
> Foo<String> foo = Foo.<String>create().withBlah(42).withBar(new
> StringBar());
>
> Example: GroupByKey does this. As well as other transforms that don't have
> a withBar()-like method, but still need a type argument, e.g. Keys.
>
> Pros: completely unambiguous, easy to code, interacts well with @AutoValue
> Cons: need to specify type once at call site.
>
> 2. Binding at a method that takes an argument of the given type (let us
> call it "a constraint argument"), e.g.:
>
> class Foo<T> {
> ...
> public static FooBuilder<?> create();
> public <U> FooBuilder<U> withBar(Bar<U> bar);
> }
>
> Foo<String> foo = Foo.create().withBlah(42).withBar(new StringBar());
>
> Example: KafkaIO
>
> https://github.com/apache/incubator-beam/blob/master/sdks/java/io/kafka/src/main/java/org/apache/beam/sdk/io/kafka/KafkaIO.java#L280
>
> Pros: don't need to specify type at call site.
> Cons: doesn't interact well with @AutoValue (it doesn't support builder
> methods that change type) - requires unchecked conversions.
>
> 3. Forcing to provide a "constraint argument" in the creation method:
>
> class Foo<T> {
> ...
> public static <T> FooBuilder<T> create(Bar<T> bar);
> // (do not provide withBar)
> }
>
> Foo<String> foo = Foo.create(new StringBar()).withBlah(42);
>
> Example: WithKeys
>
> https://github.com/apache/incubator-beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/WithKeys.java
> ,
> ParDo
>
> Pros: easy to code, interacts ok with @AutoValue, don't need to specify
> type at call site.
> Cons: need to supply all constraint arguments in the create method, so they
> are treated differently from other arguments.
>
> 4. Splitting the builder into a "bound" and "unbound" class:
>
> class Foo {
> Unbound create();
>
> class Unbound {
> public Unbound withBlah(int blah);
> public <T> Bound<T> withBar(Bar<T> bar);
> }
>
> class Bound<T> {
> public Bound<T> withBlah(int blah);
> }
> }
>
> Foo.Bound<String> foo = Foo.create().withBlah(42).withBar(new StringBar());
>
> Example: TextIO.Read
>
> https://github.com/apache/incubator-beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/io/TextIO.java
> Pros: even more type-safe at call site than the others (using an incomplete
> builder is a compile error)
> Cons: very cumbersome to implement, lots of confusion between "(un)bound"
> and "(un)bounded", tempting for clients to use ugly variable names such as
> "Foo.Bound<String> bound = ..." (rather than "foo")
>
> ****
>
> I'd like to argue in favor of #1, because:
> - It makes sense for transforms like Keys.create() which do not have a
> "constraint argument", so we can have consistency between such transforms
> and the others.
> - It is the simplest to implement, and causes the fewest amount of
> generics-related confusion when reading the implementation code.
> - It interacts well with @AutoValue builders.
>
> The only downside is that you have to specify the generic argument at call
> site, but I think this is acceptable given the benefits of consistency,
> unambiguity and providing a pattern that's easy to follow for future
> transform writers.
>
> Of course, there should be an exception for cases when there is a very
> small and fixed number of arguments, or when it's clear that the
> "constraint argument" is the most important one - e.g. ParDo.of(DoFn<A, B>)
> should *not* be changed to ParDo.<A, B>create().withFn(DoFn<A, B>). Also,
> I'm not suggesting making changes to existing transforms, only deciding
> which pattern to recommend for new transforms.
>
> WDYT?
>
