Drop into interpreter during arbitrary scala code location

Lars Yencken picture Lars Yencken · Jan 29, 2010 · Viewed 12k times · Source

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

Daniel C. Sobral picture Daniel C. Sobral · Jan 29, 2010

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:\Users\Daniel\Documents\Scala\Programas>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:\Users\Daniel\Documents\Scala\Programas>scalac TestDebugger.scala

C:\Users\Daniel\Documents\Scala\Programas>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:\Users\Daniel\Documents\Scala\Programas>