debugging - Drop into interpreter during arbitrary scala code location

Question

I come from a Python background, where at any point in my code I can add

import pdb; pdb.set_trace()

and at runtime I'll be dropped into an interactive interpreter at that spot. Is there an equivalent for scala, or is this not possible at runtime?

Answer 1

Yes, you can, on Scala 2.8. Note that, for this to work, you have to include the scala-compiler.jar in your classpath. If you invoke your scala program with scala, it will be done automatically (or so it seems in the tests I made).

You can then use it like this:

import scala.tools.nsc.Interpreter._

object TestDebugger {
  def main(args: Array[String]) {
    0 to 10 foreach { i =>
      breakIf(i == 5, DebugParam("i", i))
      println(i)
    }
  }
}

You may pass multiple DebugParam arguments. When the REPL comes up, the value on the right will be bound to a val whose name you provided on the left. For instance, if I change that line like this:

      breakIf(i == 5, DebugParam("j", i))

Then the execution will happen like this:

C:UsersDanielDocumentsScalaProgramas>scala TestDebugger
0
1
2
3
4
j: Int

scala> j
res0: Int = 5

You continue the execution by typing :quit.

You may also unconditionally drop into REPL by invoking break, which receives a List of DebugParam instead of a vararg. Here's a full example, code and execution:

import scala.tools.nsc.Interpreter._

object TestDebugger {
  def main(args: Array[String]) {
    0 to 10 foreach { i =>
      breakIf(i == 5, DebugParam("j", i))
      println(i)
      if (i == 7) break(Nil)
    }
  }
}

And then:

C:UsersDanielDocumentsScalaProgramas>scalac TestDebugger.scala

C:UsersDanielDocumentsScalaProgramas>scala TestDebugger
0
1
2
3
4
j: Int

scala> j
res0: Int = 5

scala> :quit
5
6
7

scala> j
<console>:5: error: not found: value j
       j
       ^

scala> :quit
8
9
10

C:UsersDanielDocumentsScalaProgramas>