Question

Given the following code:

case class ChangeSet(field:String, from:Object, to:Object)

private var changed:List[ChangeSet] = Nil

def change(field:String, from:Object, to:Object) {
  changed.find{ case ChangeSet(field,_,_) => true } match {
    case Some(ChangeSet(field,to,_)) => // do stuff
    case Some(_) => // do stuff
    case _ => // do stuff
  }
}

The line giving me trouble is Some(ChangeSet(field,to,_)).

It compiles but what seems to be happening is that Scala is filling it in as a placeholder for a wildcard. I base that assumption on the fact that when I do the following Some(ChangeSet(field,to,to)) I get the error to is already defined as value.

What I wanted is to make a ChangeSet object with to from the method parameters.

Is that possible?

Answer 1

When pattern matching Scala interprets all identifiers starting with lower case as placeholders and fills in values. To tell Scala to use to as a constant value from the outer scope you need to surround it with backticks: `to`. Alternatively, you could change the name of to to To as Rex Kerr suggested, but I prefer to keep my variables lowercase.

This should work:

def change(field:String, from:Object, to:Object) {
  changed.find{ case ChangeSet(`field`,_,_) => true } match {
    case Some(ChangeSet(`field`, `to`, _)) => // do stuff
    case Some(_) => // do stuff
    case _ => // do stuff
  }
}