Scala: "recursive value ... needs type" but I use Java types only

Question

object Rec extends App {
  val outStream = new java.io.ByteArrayOutputStream
  {
    val out = new java.io.PrintStream(new java.io.BufferedOutputStream(outStream))
  }
}

This seemingly simple code causes a compile error:

$ scalac rec.scala
rec.scala:2: error: recursive value out needs type
  val outStream = new java.io.ByteArrayOutputStream
                  ^
one error found

But I don't see what is "recursive."

Background: I was trying to write a unit test on println with Console.withOut

Community · Accepted Answer · 2017-05-23 11:45:00Z

4

After putting braces where they belong code looks like this:

object Rec extends App {
  val outStream = new java.io.ByteArrayOutputStream {
    val out = new java.io.PrintStream(new java.io.BufferedOutputStream(outStream))
  }
}

and this is how you create object of an anonymous class with a member out that uses the defined object recursively (outStream uses outStream in its definition).

I believe this is what you wanted to do

object Rec extends App {
  val outStream = new java.io.ByteArrayOutputStream
  val out = new java.io.PrintStream(new java.io.BufferedOutputStream(outStream))
}

If you for some reason need to create another scope, you can use locally

What does Predef.locally do, and how is it different from Predef.identity

edited May 23, 2017 at 11:45

CommunityBot

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answered Jan 26, 2016 at 12:55

Łukasz

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1 Comment

nodakai Over a year ago

Thanks for telling me about locally

slouc · Accepted Answer · 2016-01-26 12:15:24Z

4

You are assigning a value to outStream by invoking stuff to which you pass on the outStream (I marked it in CAPS). Hence the recursion.

object Rec extends App {
  val OUTSTREAM = new java.io.ByteArrayOutputStream
  {
    val out = new java.io.PrintStream(new java.io.BufferedOutputStream(OUTSTREAM))
  }
}

answered Jan 26, 2016 at 12:15

slouc

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