What on Earth is going on here?

[Paul Butcher]

With Scala 2.10.0-M3 and ScalaTest 1.8-SNAPSHOT:

Welcome to Scala version 2.10.0-M3 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_31). Type in expressions to have them evaluated.
Type :help for more information.

scala> def foo[R] = null.asInstanceOf[R] foo: [R]=> R

scala> foo[String] res0: String = null

scala> foo[Int] res1: Int = 0

scala> import org.scalatest.Suite import org.scalatest.Suite

scala> class BugSuite extends Suite { def test1 = expect(0) { foo[Int] }; def test2 = { val x = foo[Int]; expect(0) { x } } } defined class BugSuite

scala> (new BugSuite).execute()

BugSuite:

- test1 *** FAILED ***
Expected 0, but got null (<console>:9)

- test2 

Why does test1 fail? And why does introducing the temporary variable in test2 make it pass?

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[Paul Butcher]

Hmm - let's try that again, with slightly less screwed up formatting this time, I hope:

Welcome to Scala version 2.10.0-M3 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_31).

Type in expressions to have them evaluated.

Type :help for more information.

scala> def foo[R] = null.asInstanceOf[R]

foo: [R]=> R

scala> foo[String]

res0: String = null

scala> foo[Int]

res1: Int = 0

scala> import org.scalatest.Suite

import org.scalatest.Suite

scala> class BugSuite extends Suite { def test1 = expect(0) { foo[Int] }; def test2 = { val x = foo[Int]; expect(0) { x } } }

defined class BugSuite

scala> (new BugSuite).execute()

BugSuite:

- test1 *** FAILED ***

  Expected 0, but got null (<console>:9)

- test2

 

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[Vlad Patryshev]

Safe unboxing of Int to default value?

Thanks,
-Vlad

On Sat, May 19, 2012 at 1:23 PM, Paul Butcher <pa...@paulbutcher.com> wrote:

Hmm - let's try that again, with slightly less screwed up formatting this time, I hopev

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[Paul Butcher]

I'm sorry Vlad - what is "safe unboxing"? And are you suggesting that it's the cause of the problem, or a potential cure?

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[Bill Venners]

Hi Paul,

Sorry for the delay in responding. Was flying yesterday and too bushed
once I got in to investigate. I will guess that this is a corner case
of casting behavior in Scala. It smells like it could be a Scala
compiler bug but I'm not sure. It might just be trying to delay the
possibly unintended behavior of changing a null into a 0. What seems
to happen is that if an Int is needed, the null does get transformed
to a zero by the process. That's shown in these two cases:

scala> def foo[R] = null.asInstanceOf[R]
foo: [R]=> R

scala> foo[String]
res0: String = null

scala> foo[Int]
res1: Int = 0

scala> val x = foo[Int]
x: Int = 0

scala> val y: Int = foo[Int]
y: Int = 0

I'm guessing that's behaving as specified, because the other possible
behavior is a NullPointerException, which was a problem with Java's
autoboxing. That does make sense, but frankly magically transforming a
null to an 0 might also cause bugs.

ScalaTest's except's second param is an Any, and that's where you were
seeing the persistent null, so I tried this:

scala> val z: Any = foo[Int]
z: Any = null

Same behavior you saw with except. If it doesn't need an Int yet, then
it stays null. You might want to ask the scala-internals list if this
is specified behavior. It might be, but worth asking.

Bill

Bill Venners
Artima, Inc.
http://www.artima.com

[Vlad Patryshev]

Paul,

I believe Bill has explained it in details below.

Thanks,
-Vlad

[Paul Butcher]

This turns out to be a particularly wormy can:

https://groups.google.com/d/topic/scala-internals/7fGPoRGfflM/discussion

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