The Syntax of Capture Checking?

Question
#1

Hi,

I tried the sample code from Capture Checking with the bightly build 3.4.0-RC1-bin-20230804-73967d5-NIGHTLY. I got the following error:

Compiling project (Scala 3.4.0-RC1-bin-20230804-73967d5-NIGHTLY, JVM)
[error] ./CC.scala:14:27
[error] Illegal start of declaration
[error] def usingLogFile[T](op: ({*} FileOutputStream) => T): T =
[error]                           ^
[error] ./CC.scala:14:46
[error] an identifier expected, but ')' found
[error] def usingLogFile[T](op: ({*} FileOutputStream) => T): T =
[error]                                              ^
Error compiling project (Scala 3.4.0-RC1-bin-20230804-73967d5-NIGHTLY, JVM)

For the experimental language feature, is the syntaxt different from the document?

Thanks for your help!

Guofeng

#2

Did you enable that language.experimental.captureChecking flag?

#3

The syntax have changed, but it was not reflected in the docs

#4

It works following the changed cc sytax.

But the following code pass the compilation:

val later = usingLogFile { f => () => f.write(0) }

and got the following runtime error:

 ../scala-cli -S 3.4.0-RC1-bin-20230804-73967d5-NIGHTLY CC.scala

Compiling project (Scala 3.4.0-RC1-bin-20230804-73967d5-NIGHTLY, JVM)
Compiled project (Scala 3.4.0-RC1-bin-20230804-73967d5-NIGHTLY, JVM)
Exception in thread "main" java.io.IOException: Stream Closed
	at java.base/java.io.FileOutputStream.write(Native Method)
	at java.base/java.io.FileOutputStream.write(FileOutputStream.java:322)
	at CC$package$.$anonfun$1$$anonfun$1(CC.scala:11)
	at CC$package$.shouldGetAStaticErrorAsDocumented(CC.scala:12)
	at CC$package$.CC(CC.scala:19)
	at CC.main(CC.scala:14)

The following is the code I use:

import java.io.FileOutputStream
import language.experimental.captureChecking

def usingLogFile[T](op: FileOutputStream^ => T): T =
  val logFile = FileOutputStream("log")
  val result = op(logFile)
  logFile.close()
  result

def shouldGetAStaticErrorAsDocumented =
  val later = usingLogFile { file => (x:Int) => file.write(x) }
  later(0) // crash

@main def CC: Unit =
  val xs = usingLogFile { f =>
    List(1, 2, 3).map { x => f.write("x\n".getBytes()); x * x }
  }
  
  shouldGetAStaticErrorAsDocumented

It is ok to me. I just want to confirm here if it is just an issue of the NIGHTLY Scala 3 compiler.

Thanks for all your helps!

Guofeng

#5

I don’t think this is definitive, but the current version of the capture checker requires the use of sealed on the type parameter to prevent impure functions from escaping:
def usingLogFile[sealed T](op: FileOutputStream^ => T): T =

11 |  val later = usingLogFile { file => (x:Int) => file.write(x) }
   |              ^^^^^^^^^^^^
   |Sealed type variable T cannot be instantiated to box (x$0: Int) => Unit since
   |that type captures the root capability `cap`.
   |This is often caused by a local capability in an argument of method usingLogFile
   |leaking as part of its result.