Question

I’m very new to Scala, and while reading the ScalaTutorial.pdf Section 6: Case classes and pattern matching

I find no information on how to run its example which is:

package my

abstract class Tree
case class Sum(l: Tree, r: Tree) extends Tree
case class Var(n: String) extends Tree 
case class Const(i: Int) extends Tree 

object TestTree {

  type Environment = String => Int

  def eval(t: Tree, env: Environment): Int = t match {
    case Sum(l, r) => eval(l, env) + eval(r, env)
    case Var(n) => env(n)
    case Const(v) => v
  }

  def main(args: Array[String]){

    val s : Sum = Sum(Var("x"), Const(10))
    // Then how to define a variable of type environment to pass it to the `eval` function:
    //eval(s, Environment) ??
  }
}

I don’t know how to pass the Environment to the eval function

Answer 1

type Environment = String => Int

This says that the type Environment is equal to the type String => Int, i.e. the type of functions that take a String and return an Int. It should be noted that in Scala a map is a kind of function (which is to say Map[K,V] is a subtype of K => V). So any function that takes a function as an argument can also take a map instead.

So to use eval you can pass it a function of type String => Int, which might either be an actual function that you defined and that takes a string and returns an int, or a map that maps strings to integers.