NPE when running command line args with `extends App`

[spamegg1]

Compiler version

3.1.2

Minimized code

Taken from the docs

// in file HelloYou.scala
object HelloYou extends App {
    if (args.size == 0)
        println("Hello, you")
    else
        println("Hello, " + args(0))
}

Compile with scalac HelloYou.scala then run with scala HelloYou Spam

Output

Compilation succeeds, but running scala HelloYou Spam crashes:

CLICK TO EXPAND

 ➜ scala HelloYou Spam
java.lang.NullPointerException
	at java.base/java.lang.reflect.Array.getLength(Native Method)
	at scala.collection.ArrayOps$.size$extension(ArrayOps.scala:202)
	at Hello$.<clinit>(Hello.scala:2)
	at Hello.main(Hello.scala)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at dotty.tools.runner.RichClassLoader$.run$extension$$anonfun$1(ScalaClassLoader.scala:36)
	at dotty.tools.runner.ScalaClassLoader$.asContext(ScalaClassLoader.scala:80)
	at dotty.tools.runner.RichClassLoader$.dotty$tools$runner$RichClassLoader$$$asContext$extension(ScalaClassLoader.scala:18)
	at dotty.tools.runner.RichClassLoader$.run$extension(ScalaClassLoader.scala:36)
	at dotty.tools.runner.CommonRunner.run(ObjectRunner.scala:23)
	at dotty.tools.runner.CommonRunner.run$(ObjectRunner.scala:13)
	at dotty.tools.runner.ObjectRunner$.run(ObjectRunner.scala:48)
	at dotty.tools.runner.CommonRunner.runAndCatch(ObjectRunner.scala:30)
	at dotty.tools.runner.CommonRunner.runAndCatch$(ObjectRunner.scala:13)
	at dotty.tools.runner.ObjectRunner$.runAndCatch(ObjectRunner.scala:48)
	at dotty.tools.MainGenericRunner$.run$1(MainGenericRunner.scala:212)
	at dotty.tools.MainGenericRunner$.main(MainGenericRunner.scala:267)
	at dotty.tools.MainGenericRunner.main(MainGenericRunner.scala)

Expectation

scala HelloYou Spam should work correctly and print Hello, Spam.

Works as expected when compiled and run with Scala 2.

[bishabosha]

Thank you for reporting, however this is known to fail: https://docs.scala-lang.org/scala3/reference/dropped-features/delayed-init.html

[bishabosha]

Also, I noticed that the example you used is from the Scala 2 book, the equivalent for Scala 3 is here: https://docs.scala-lang.org/scala3/book/taste-hello-world.html

[spamegg1]

OK, I also thought that it's probably not a bug, but changed in Scala 3. Weird, it still does compile though! So it's natural to expect it to work.

Just to mention: some newcomers come across these examples in the docs, and they don't know the difference between Scala 2 and 3, or why a piece of code (a Hello world example at that!) straight from the official docs fails. It's not a good look for the language! The docs page starts with Scala 2, and you have to scroll all the way down to see Scala 3.

[bishabosha]

Just to mention: some newcomers come across these examples in the docs, and they don't know the difference between Scala 2 and 3, or why a piece of code (a Hello world example at that!) straight from the official docs fails. It's not a good look for the language! The docs page starts with Scala 2, and you have to scroll all the way down to see Scala 3.

We definitely hear your concerns so hopefully the situation will begin to improve soon with scala/docs.scala-lang#2430