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:\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>