REPL attempts infinite iterable toString

[som-snytt]

Minimized code

scala> val vs = new Iterable[Int] { def iterator = Iterator.continually(42) }

Output

java.lang.OutOfMemoryError: Java heap space
  at java.base/java.util.Arrays.copyOf(Arrays.java:3536)
  at java.base/java.lang.AbstractStringBuilder.ensureCapacityInternal(AbstractStringBuilder.java:228)
  at java.base/java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:582)
  at java.base/java.lang.StringBuilder.append(StringBuilder.java:173)
  at java.base/java.lang.StringBuilder.append(StringBuilder.java:167)
  at scala.collection.IterableOnceOps.addString(IterableOnce.scala:1187)
  at scala.collection.IterableOnceOps.addString$(IterableOnce.scala:1179)
  at rs$line$1$$anon$1.addString(rs$line$1:1)
  at scala.collection.IterableOnceOps.mkString(IterableOnce.scala:1129)
  at scala.collection.IterableOnceOps.mkString$(IterableOnce.scala:1127)
  at rs$line$1$$anon$1.mkString(rs$line$1:1)
  at scala.collection.Iterable.toString(Iterable.scala:76)
  at scala.collection.Iterable.toString$(Iterable.scala:76)
  at rs$line$1$$anon$1.toString(rs$line$1:1)
  at scala.runtime.ScalaRunTime$.inner$1(ScalaRunTime.scala:255)
  at scala.runtime.ScalaRunTime$.stringOf(ScalaRunTime.scala:266)
  at scala.runtime.ScalaRunTime$.replStringOf(ScalaRunTime.scala:274)
  at scala.runtime.ScalaRunTime.replStringOf(ScalaRunTime.scala)
  at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:64)
  at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.base/java.lang.reflect.Method.invoke(Method.java:564)
  at dotty.tools.repl.Rendering.classLoader$$anonfun$1(Rendering.scala:62)
  at dotty.tools.repl.Rendering$$Lambda$1329/0x00000008010a22f8.apply(Unknown Source)
  at dotty.tools.repl.Rendering.replStringOf(Rendering.scala:71)
  at dotty.tools.repl.Rendering.$anonfun$4(Rendering.scala:85)
  at dotty.tools.repl.Rendering$$Lambda$1332/0x00000008010a3630.apply(Unknown Source)
  at scala.Option.map(Option.scala:242)
  at dotty.tools.repl.Rendering.valueOf(Rendering.scala:85)
  at dotty.tools.repl.Rendering.renderVal(Rendering.scala:121)
  at dotty.tools.repl.ReplDriver.$anonfun$13(ReplDriver.scala:308)
  at dotty.tools.repl.ReplDriver$$Lambda$1308/0x0000000801095328.apply(Unknown Source)

Expectation

I might want to use that iterable.

This is a transfer of scala/bug#11785 which was not previously resolved.

[griggt]

It seems that this is because the call to useOwnToString returns true in:

    def inner(arg: Any): String = arg match {
      case null                         => "null"
      case ""                           => "\"\""
      case x: String                    => if (x.head.isWhitespace || x.last.isWhitespace) "\"" + x + "\"" else x
      case x if useOwnToString(x)       => x.toString     // *** HERE this case matches, we never get to Iterable[_] below
      case x: AnyRef if isArray(x)      => arrayToString(x)
      case x: scala.collection.Map[_, _] => x.iterator.take(maxElements).map(mapInner).mkString(x.collectionClassName + "(", ", ", ")")
      case x: Iterable[_]               => x.iterator.take(maxElements).map(inner).mkString(x.collectionClassName + "(", ", ", ")")
      case x: Product1[_] if isTuple(x) => "(" + inner(x._1) + ",)" 
      case x: Product if isTuple(x)     => x.productIterator.map(inner).mkString("(", ",", ")")
      case x                            => x.toString
    }

https://github.com/scala/scala/blob/51790d7a5b0b1c12a16e14a1bd65c1f0941dda47/src/library/scala/runtime/ScalaRunTime.scala#L251-L262

[som-snytt]

Thanks, I totally forgot this ticket. Probably the events of Jan 6 erased my memory.

[griggt]

Ref: scala/bug#3710
scala/scala@11ae7ea

It seems that the logic is intended, in part, to prevent infinite traversal, and here is having the opposite effect.

[som-snytt]

Thanks for tracking that down. The best part is

    val maxElements = 10000

So the real bug is that a zero was lost at some point.

[som-snytt]

I returned to this because the Scala 3 REPL currently (recently) asks for both a bounded and unbounded string result from stringOf in the runtime, in order to detect whether the output was truncated. It occurred to me that an unbounded result must invite infinite strings.

In this case, indeed, custom Iterable is expected to supply a custom toString.

Recently, I wondered if there is a use case for isTraversableAgain: maybe it implies finitely traversable.

I will close this and bounce it back to the Scala 2 ticket.