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https://issues.apache.org/jira/browse/FLINK-2809?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14961835#comment-14961835
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ASF GitHub Bot commented on FLINK-2809:
---------------------------------------
Github user aalexandrov commented on the pull request:
https://github.com/apache/flink/pull/1217#issuecomment-148901986
> Not all methods without paremeters should translate to methods without
parenthesis...
@StephanEwen I agree with that, but I cannot understand how the
`UnitTypeInfo` might cause a confusion here.
The typeInformation macros are synthesized by the macro based on the
inferred collection type, which means that the meaning of `()` is resolved
before that. Consider the following example:
```scala
// in the Scala REPL
case class Foo(answer: Int)
// defined class Foo
def f1(): Foo = Foo(42)
// f1: ()Foo
def f2: Foo = Foo(42)
// f2: Foo
val xs = Seq(f1(), f2) // how a literate person would write it
// xs: Seq[Foo] = List(Foo(42), Foo(42))
val xs = Seq(f1, f2) // how a dazed & confused person would write it, but
still compiles
// xs: Seq[Foo] = List(Foo(42), Foo(42))
val xs = Seq(f1, f2()) // even worse, but this breaks with a compiler
exception
// error: Foo does not take parameters
// val xs = Seq(f1, f2())
val xs = Seq((), ()) // typing '()' without syntactic context resolves to
Unit
// xs: Seq[Unit] = List((), ())
```
In all of the above situations `env.fromCollection(xs)` is (1) either going
to typecheck and trigger `TypeInformation` synthesis or (2) fail with the above.
Can you point to StackOverflow conversation or something similar where the
issue you mention is explained with an example?
> DataSet[Unit] doesn't work
> --------------------------
>
> Key: FLINK-2809
> URL: https://issues.apache.org/jira/browse/FLINK-2809
> Project: Flink
> Issue Type: Bug
> Components: Scala API
> Reporter: Gabor Gevay
> Assignee: Gabor Gevay
> Priority: Minor
>
> The following code creates a DataSet\[Unit\]:
> val env = ExecutionEnvironment.createLocalEnvironment()
> val a = env.fromElements(1,2,3)
> val b = a.map (_ => ())
> b.writeAsText("/tmp/xxx")
> env.execute()
> This doesn't work, because a VoidSerializer is created, which can't cope with
> a BoxedUnit. See exception below.
> I'm now thinking about creating a UnitSerializer class.
> org.apache.flink.runtime.client.JobExecutionException: Job execution failed.
> at
> org.apache.flink.runtime.jobmanager.JobManager$$anonfun$receiveWithLogMessages$1.applyOrElse(JobManager.scala:314)
> at
> scala.runtime.AbstractPartialFunction.apply(AbstractPartialFunction.scala:36)
> at
> org.apache.flink.runtime.ActorLogMessages$$anon$1.apply(ActorLogMessages.scala:36)
> at
> org.apache.flink.runtime.ActorLogMessages$$anon$1.apply(ActorLogMessages.scala:29)
> at scala.PartialFunction$class.applyOrElse(PartialFunction.scala:123)
> at
> org.apache.flink.runtime.ActorLogMessages$$anon$1.applyOrElse(ActorLogMessages.scala:29)
> at akka.actor.Actor$class.aroundReceive(Actor.scala:465)
> at
> org.apache.flink.runtime.jobmanager.JobManager.aroundReceive(JobManager.scala:92)
> at akka.actor.ActorCell.receiveMessage(ActorCell.scala:516)
> at akka.actor.ActorCell.invoke(ActorCell.scala:487)
> at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:254)
> at akka.dispatch.Mailbox.run(Mailbox.scala:221)
> at akka.dispatch.Mailbox.exec(Mailbox.scala:231)
> at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
> at
> scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
> at
> scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
> at
> scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)
> Caused by: java.lang.ClassCastException: scala.runtime.BoxedUnit cannot be
> cast to java.lang.Void
> at
> org.apache.flink.api.common.typeutils.base.VoidSerializer.serialize(VoidSerializer.java:26)
> at
> org.apache.flink.runtime.plugable.SerializationDelegate.write(SerializationDelegate.java:51)
> at
> org.apache.flink.runtime.io.network.api.serialization.SpanningRecordSerializer.addRecord(SpanningRecordSerializer.java:76)
> at
> org.apache.flink.runtime.io.network.api.writer.RecordWriter.emit(RecordWriter.java:83)
> at
> org.apache.flink.runtime.operators.shipping.OutputCollector.collect(OutputCollector.java:65)
> at
> org.apache.flink.runtime.operators.chaining.ChainedMapDriver.collect(ChainedMapDriver.java:78)
> at
> org.apache.flink.runtime.operators.DataSourceTask.invoke(DataSourceTask.java:177)
> at org.apache.flink.runtime.taskmanager.Task.run(Task.java:564)
> at java.lang.Thread.run(Thread.java:745)
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