For http://stackoverflow.com/questions/12699372/scala-interpreter

P.scala

object X extends App {
  val s = StatementParser("x = (x + 1)")
  println(s)
}

trait Expression
case class Plus(a: Expression, b: Expression) extends Expression
case class Var(name: String) extends Expression
case class Num(n: Int) extends Expression
trait Statement
case class Assign(v: String, ex: Expression) extends Statement

import scala.util.parsing.combinator.RegexParsers

object StatementParser extends RegexParsers {
  private lazy val num: Parser[Num] =
    """(\d+)""".r ^^ (n => Num(n.toInt))

  private lazy val identifier: Parser[String] =
    """([A-Za-z][A-Za-z0-9]*)""".r

  private lazy val variable: Parser[Var] =
    identifier ^^ Var

  private lazy val expression: Parser[Expression] =
    plus | variable | num

  private lazy val plus: Parser[Plus] =
    ("(" ~> expression <~ "+") ~ expression <~ ")" ^^ { case l ~ r => Plus(l, r) }

  private lazy val assign: Parser[Assign] =
    (identifier <~ "=") ~ expression ^^ { case v ~ ex => Assign(v, ex) }

  def apply(str: String): Statement = parseAll(assign, str) match {
    case Success(result, _) => result
    case NoSuccess(msg, _) => sys.error("Could not parse the input string: "+msg)
  }
}