pattern matching - Scala match recursive objects

Question

I am a newbie in Scala and I want to build a small recursive pattern matching with objects. I keep getting a No match in the following code:

abstract class FixedNode

case class LoopBeginNode(lb: ArrayLengthNode) extends FixedNode  
case class ArrayLengthNode(al: FixedNode) extends FixedNode  
case class IfNode(i: FixedNode) extends FixedNode
case class BeginNode(b: FixedNode) extends FixedNode
case class LoadIndexedNode(li: FixedNode) extends FixedNode
case class FixedGuardNode(fg: FixedNode) extends FixedNode
case class EndNode(e: String) extends FixedNode

object HelloWorld {
    
    def main(args: Array[String]) {
      println(matchIt(LoopBeginNode(ArrayLengthNode(EndNode("")))))
    }
    
    def matchIt(e: FixedNode): String = e match {
      case EndNode(en) => "match"
      case LoopBeginNode(lb) if lb == ArrayLengthNode(e) => matchIt(lb)
      case ArrayLengthNode(al) if al == EndNode("") => matchIt(al)
      case IfNode(i) if i == BeginNode(i) => matchIt(i)
      case BeginNode(b) if b == EndNode("") =>matchIt(b)
      case _ => "No match"
    }
   }

If I add a simple LoopBeginNode(EndNode("")) I get a match. Although in the more complex case above I get a mismatch, which according to my understanding, comes from the lb variable which is an EndNode whereas in the condition is a FixedNode. Do you have any idea how I am going to make it work? I want in the end to conclude in something more complex like

ArrayLengthNode(IfNode(BeginNode(LoadIndexedNode(GuardeElseNode(LoadIndexNode(IfNode(EndNode(""))))))))

question from:https://stackoverflow.com/questions/65924782/scala-match-recursive-objects

Answer 1

Your pattern matching construction was not really correct, please find fixed one below:

/**
* Use `sealed` to box type and pattern matching will show compilation warnings.
*/
sealed trait FixedNode

case class LoopBeginNode(lb: ArrayLengthNode) extends FixedNode  
case class ArrayLengthNode(al: FixedNode) extends FixedNode  
case class IfNode(i: FixedNode) extends FixedNode
case class BeginNode(b: FixedNode) extends FixedNode
case class LoadIndexedNode(li: FixedNode) extends FixedNode
case class FixedGuardNode(fg: FixedNode) extends FixedNode
case class EndNode(e: String) extends FixedNode

object HelloWorld {
    
    def main(args: Array[String]) {
      println(matchIt(LoopBeginNode(ArrayLengthNode(EndNode("")))))
      
      //ArrayLengthNode(IfNode(BeginNode(LoadIndexedNode(GuardeElseNode(LoadIndexNode(IfNode(EndNode(""))))))))
    }
    
    def matchIt(e: FixedNode): String = e match {
      case EndNode(en) => "match"
      /**
       * In previous version `case LoopBeginNode(lb) if lb == ArrayLengthNode(e) => matchIt(lb)`
       * 'lb == ArrayLengthNode(e)' - always false, because you comparing in this case 
       * `ArrayLengthNode(EndNode("")) == ArrayLengthNode(LoopBeginNode(ArrayLengthNode(EndNode(""))))`
       * which is always false.
       *
       * Same approach used for other patterns
       */
      case LoopBeginNode(ArrayLengthNode(lb)) => matchIt(lb)
      case ArrayLengthNode(EndNode("")) => "match"
      case IfNode(BeginNode(i)) => matchIt(i)
      case BeginNode(EndNode("")) => "match"
      case _ => s"No match at: `$e`"
    }
}

And Scatie to play with it: https://scastie.scala-lang.org/zfEd5DprQUKDttLcc6D54w