Question

I want to run the class Expr below. I’ve taken this code from http://andymaleh.blogspot.ie/2008/04/scalas-pattern-matching-visitor-pattern.html

Here is what I am trying :

import Expr.Sum

object Main {

    def main(args:Array[String]) {

      var expr1 = new Sum(new Num(1), new Prod(new Num(2), new Num(3)))
      print(expr1)
  }

}

abstract class Expr {

  case class Num(n: Int) extends Expr
  case class Sum(l: Expr , r: Expr) extends Expr
  case class Prod(l: Expr, r: Expr) extends Expr

  def evalExpr(e: Expr): Int = e match {
    case Num(n) => n
    case Sum(l, r) => evalExpr(l) + evalExpr(r)
    case Prod(l, r) => evalExpr(l) * evalExpr(r)
  }

  def printExpr(e: Expr) : Unit = e match {
    case Num(n) => print(" " + n + " ")
    case Sum(l, r) => printExpr(l); print("+"); printExpr(r)
    case Prod(l, r) => printExpr(l); print("x"); printExpr(r)
  }

}

But line

import Expr.Sum

is causing a compile time error : ‘not found: object Expr’ . How can I import the class Expr ?

Answer 1

You can only import members of instances. So your code would be:

object Main {
    def main(args:Array[String]) {
      val expr = new Expr {} // now we have a instance.

      import expr._
      var expr1 = new Sum(new Num(1), new Prod(new Num(2), new Num(3)))
      print(expr1)
  }
}

A simple example explains why you can’t import non-instances members:

class A(val x:Int) {
  object Print { def print = println(x) }
}

If you could import A.Print, which value would be bound to x? Now if we do:

val a = new A(5); // we have a value bound to x.
import a._
Print.print

This is one reason. The other reason that new A(5).Print != new A(5).Print, not only in equality, but also in type: val (a1, a2) = (new A(5), new A(5)); implicitly[a1.Print <:< a2.Print] wouldn’t compile. This is what Scala calls path-dependent types.