Case Classes and Pattern Matching on List

Question

What I would like to do is simplify this expression

Opt(Opt(ExpList(List(Opt(Opt(Var("optional")))))))

to get something like that

Opt(ExpList(List(Opt(Var("optional")))))

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What would match expression look like in order to simplify all expressions inside the list.

What are best-practises for those kind of tasks?

The code snippet I tried is this:

object CaseClassPatternMatching extends App {
  abstract class Expr
  case class Var(name: String) extends Expr
  case class Opt(expr: Expr) extends Expr
  case class ExpList(listExp: List[Expr]) extends Expr

  def joinOpt(feature: Expr): Expr = feature match {
    case Opt(Opt(f)) => joinOpt(Opt(f))    // Opt(Opt("test")) --> Opt("test")
    // case ExpList(list) => ????          // What to do there?
    case _ => feature
  }

  val expr1: Expr = joinOpt(Opt(Opt(Opt(Var("optional")))))
  println(Opt(Var("optional"))) 
  // Output: Opt(Var(optional))  --> That one is OK...

  val expr2: Expr = joinOpt(Opt(Opt(ExpList(List(Opt(Opt(Var("optional"))))))))
  println(expr2)
  // Output: Opt(ExpList(List(Opt(Opt(Var(optional))))))  --> Not OK...
  // How to simplify expressions inside list?
}

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[EDIT]

For those of you who are interested, similar topic:

Scala case classes, pattern matching and varargs

Answer 1

You need four cases:

def joinOpt(feature: Expr): Expr = feature match {
                         // remove extra Opt 
                         // (you can use @ to avoid recreating Opt)
  case Opt(opt @ Opt(_)) => joinOpt(opt) 
                         // preserve single Opt
  case Opt(expr)         => Opt(joinOpt(expr)) 
                         // apply function to all elements in inner list
  case ExpList(list)     => ExpList(list map joinOpt) 
  case _ => feature
}