how to check errors happening inside scala interpreter programatically

Question

I am executing scala code snippets using scala.tools.nsc.Interpreter When the code snippet is correct, everything is fine, but when it is buggy, my code is unable to find out and happily goes on. I would like to get an exception or method to call to get any error that happened during last evaluation by the interpreter.

my code:

import scala.tools.nsc.Interpreter
import scala.tools.nsc.Settings
object RuntimeEval {
  def main(args: Array[String]): Unit = {
    var msg = "fail"
    val eval = new RuntimeEval
    msg = eval.eval(msg,"\"success\"")
    println(msg)
    var anInt = 0
    while(true){
    println("Enter an integer")
      val userInput = Console.readLine
      anInt = eval.eval(anInt,userInput)
      println("-->"+anInt)
      println
    }
  }
}
class ResContainer(var value: Any)
class RuntimeEval {
  val settings = new Settings
  settings.classpath.value = System.getProperty("java.class.path")
  val interp = new Interpreter(settings)

  def eval[A <: Any](obj: A, expression: String): A={
    val res = new ResContainer(obj)
    interp.beQuietDuring {
      interp.bind("res", res.getClass.getCanonicalName, res)
      interp.interpret("res.value = "+expression)
    }
    val info = obj match{
      case x: AnyRef => "expected type: \n"+x.getClass.getCanonicalName+"\n"
      case _ => "expected type is not an AnyRef\n"
    }
    res.value match{
      case x: A => x
      case x: AnyRef => error("unexpected result type, "+info+"result type:\n"+x.getClass.getCanonicalName)
      case _ => error("unexpected result type, "+info+"result type is not an AnyRef")
    }
  }
}

an example of the problem:

success
Enter an integer
9/12
-->0

Enter an integer
9/0
java.lang.ArithmeticException: / by zero
    at .<init>(<console>:6)
    at .<clinit>(<console>)
    at RequestResult$.<init>(<console>:5)
    at RequestResult$.<clinit>(<console>)
    at RequestResult$scala_repl_result(<console>)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at scala.tools.nsc.Interpreter$Request$$anonfun$loadAndRun$1$$anonfun$apply$17.apply(Interpreter.scala:988)
    at scala.tools.nsc.Interpreter$Request$$anonfun$loadAndRun$1$$anonfun$apply$17.apply(Interpreter.scala:988)
    at scala.util.control.Exception$Catch.apply(Exception.scala:79)
    at scala...-->0

Enter an integer
9/4
-->2

Enter an integer

The ArithmeticException happened inside the interpreter, then it seems it returned nothing, so my code got the same result as previous operation, 0. How to catch this ?

score 7 · Accepted Answer · answered Jul 24 '11 at 14:19

The interpret method of the Interpreter returns a result value which indicates if the code could be successfully interpreted or not. Thus:

interp.beQuietDuring {
  interp.bind("res", res.getClass.getCanonicalName, res)
  interp.interpret("res.value = "+expression)
} match {
   case Results.Error => error( ... )
   case Results.Incomplete => error( ... )
   case Results.Success => res.value
}

Two more things: You don't need to bind "res" for every eval, it should be sufficient to do that once you initialize the interpreter. Also, note that there is a method mostRecentVar, so you may do away with the result binding altogether. Here is an example for Scala 2.9 (same as 2.8, but instead of Interpreter you use IMain):

import tools.nsc.interpreter.{IMain, Results}
import sys.error
val interp = new IMain()
def eval( expression: String ) : AnyRef =
   interp.interpret( expression ) match {
      case Results.Error => error( "Failed" )
      case Results.Incomplete => error( "Incomplete" )
      case Results.Success => interp.valueOfTerm( interp.mostRecentVar )
        .getOrElse( error( "No result" ))
   }

Testing:

scala> val x = eval( "1 + 2" )
res0: Int = 3
x: AnyRef = 3

thanks a lot, the Scala 2.9 version works well and is more convenient. Subsequent question: how to get the original error message ("/ by 0" instead of just "Failed") — acapola, Jul 24 '11 at 16:06
I think you can wrap your expression in a `try-catch` block maybe? like `interpret( "try { " + expression + " } catch { case e => e }" )`. That way you could test whether the result is a `Throwable`? — 0__, Jul 24 '11 at 17:50
This is great, thank you. Unfortunately, it seems to have broken with scala-2.9.1. In particular, interp.valueOfTerm() seems to always give None(). :( https://issues.scala-lang.org/browse/SI-4899 — wmacura, Oct 17 '11 at 18:35
@0__ I am stuck in issue related to interpreter https://stackoverflow.com/questions/52965997/internalcompilerexception-compiling-class-was-loaded-through-a-different-loader please help me out to solve this — user811602, Oct 24 '18 at 13:01

how to check errors happening inside scala interpreter programatically

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