shaktimaan shaktimaan - 29 days ago 5
Scala Question

Displaying nested arithmetic expressions as a string using Scala pattern matching

I am learning the concepts of pattern matching in Scala. Following is an exercise for the same. The task is to define a show function that outputs an expression as a

String
. Following are the definitions involved:

object Test {
trait Expr
case class Number(n: Int) extends Expr
case class Sum(e1: Expr, e2: Expr) extends Expr
case class Prod(e1: Expr, e2: Expr) extends Expr
case class Var(name: String) extends Expr

def nestOperations(e1: Expr, e2: Expr): String = {
val l = e1 match {
case Number(n) => n.toString
case Prod(x, y) => nestOperations(x, y)
case Var(s) => s
case Sum(x, y) => "(" + show(x) + " + " + show(y) + ")"
}
val r = e2 match {
case Number(n) => n.toString
case Prod(x, y) => nestOperations(x, y)
case Var(s) => s
case Sum(x, y) => "(" + show(x) + " + " + show(y) + ")"
}
l + " * " + r
}

def show(e: Expr): String = e match {
case Number(n) => n.toString
case Sum(e1, e2) => show(e1) + " + " + show(e2)`
case Prod(e1, e2) => nestOperations(e1, e2)
case Var(s) => s
}
show(Sum(Number(1), Number(2)))
show(Sum(Prod(Number(2), Var("x")), Var("y")))
show(Prod(Sum(Number(2), Var("x")), Var("y")))
}


The expectation is that:
show(Sum(Number(1), Number(2)))
outputs
1 + 2

show(Sum(Prod(Number(2), Var("x")), Var("y")))
outputs
2 * x + y

show(Prod(Sum(Number(2), Var("x")), Var("y")))
outputs
(2 + x) * y


The program achieves all of the above. I want to know if the
nestOperations
method can be simplified? Looks like a lot of repetitive code in there.

Answer

You're right. That repeated code in nestOperations() can be eliminated.

def nestOperations(es: Expr*): String =
  es.map {
    case Number(n) => n.toString
    case Prod(x, y) => nestOperations(x, y)
    case Var(s) => s
    case Sum(x, y) => "(" + show(x) + " + " + show(y) + ")"
  }.mkString(" * ")

But why stop there? You can pull much the same stunt with show() and eliminate nestOperations() altogether.

def show(es: Expr*): String = es.map{
  case Number(n) => n.toString
  case Sum(e1, e2) => if (es.length > 1) "(" + show(e1) + " + " + show(e2) + ")"
                      else show(e1) + " + " + show(e2)
  case Prod(e1, e2) => show(e1, e2)
  case Var(s) => s
}.mkString(" * ")