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 ?

Answer 1

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