Embedded Scala REPL Interpreter example for 2.10

Question

Please give a code example of how to create an embedded Scala REPL interpreter programmatically, that works in Scala 2.10. (I added this Q&A after spending hours combing various code scraps to get a working interpreter)

Answer 1

Example Repl.scala:

import scala.tools.nsc.interpreter._
import scala.tools.nsc.Settings


object Repl extends App {
  def repl = new ILoop {
    override def loop(): Unit = {
      intp.bind("e", "Double", 2.71828)
      super.loop()
    }
  }

  val settings = new Settings
  settings.Yreplsync.value = true


  //use when launching normally outside SBT
  settings.usejavacp.value = true      

  //an alternative to 'usejavacp' setting, when launching from within SBT
  //settings.embeddedDefaults[Repl.type]

  repl.process(settings)
}

Some notes

• I choose to show the JLineReader (default) rather than SimpleReader because it works much better, correctly handling arrow keys, delete etc. JLine does add an jar dependency.
• The example shows how to bind values into the repl (variable e above).
• When I omit settings.Yreplsync.value = true, the REPL hangs and is useless.
• From my testing, if both usejavacp and embeddedDefaults settings are combined together, an error results

I find this easiest to test via SBT; a sample build.sbt:

name := "Repl"

organization := "ExamplesRUs"

scalaVersion := "2.10.2"

libraryDependencies ++= Seq(
 "org.scala-lang" % "scala-compiler" % "2.10.2",
 "org.scala-lang" % "jline" % "2.10.2"
)

Sample SBT session:

> run-main Repl
[info] Running Repl
Welcome to Scala version 2.10.2 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_37).
Type in expressions to have them evaluated.
Type :help for more information.
e: Double = 2.71828

scala> 2 * e
res1: Double = 5.43656

scala>

Answer 2

Based on Ben's excellent answer, below is a helper class to ease starting the interpreter. Usage:

Repl.run(("e", "Double", 2.71828), ("pi", "Double", 3.1415))

It automatically detects when you're running from SBT and accommodates.

Repl.scala:

import scala.tools.nsc.interpreter.ILoop
import scala.tools.nsc.Settings
import java.io.CharArrayWriter
import java.io.PrintWriter

object Repl {

  def run(params: (String, String, Any)*) {

    def repl = new ILoop {
      override def loop(): Unit = {
        params.foreach(p => intp.bind(p._1, p._2, p._3))
        super.loop()
      }
    }

    val settings = new Settings
    settings.Yreplsync.value = true

    // Different settings needed when running from SBT or normally
    if (isRunFromSBT) {
      settings.embeddedDefaults[Repl.type]
    } else {
      settings.usejavacp.value = true
    }

    repl.process(settings)
  }

  def isRunFromSBT = {
    val c = new CharArrayWriter()
    new Exception().printStackTrace(new PrintWriter(c))
    c.toString().contains("at sbt.")
  }

}