first code in scala language

test.scala

package Hodza

abstract class Expr() {}
class TrueExpr extends Expr {}
class FalseExpr extends Expr {}

class Or[T <: Expr,U <: Expr](val left: T, val right:U) extends Expr {}

trait Evaluator[T] {
  def eval(expr:T): Boolean
}
object EvalSimple{
  implicit object TrueEvaluator extends Evaluator[TrueExpr] {
    def eval(expr : TrueExpr): Boolean = true
  }
  implicit object FalseEvaluator extends Evaluator[FalseExpr] {
    def eval(expr : FalseExpr): Boolean = false
  }
}
import EvalSimple._
object EvalOr{
  implicit def orToEvaluator[A <: Expr, B <: Expr](implicit a: Evaluator[A], b: Evaluator[B]) : Evaluator[Or[A, B]] = {
    new Evaluator[Or[A, B]] {
      def eval(expr : Or[A, B]):Boolean = a.eval(expr.left) || b.eval(expr.right)
    }
  }
}
import EvalOr._
object HelloWorld {
    def evla[T](t: T) (implicit m: Evaluator[T]) : Boolean = m.eval(t)
    def main(args: Array[String]) {
      println(evla(new Or(new FalseExpr(), new FalseExpr())))
    }
  }