Question

I’ve written a visitor pattern and I’m matching an overridden subclass. I want to add the variable to the set value of the object and then return the modified object. How can I do that syntactically?

trait PropositionOrderer extends Visitor[Proposition]{
  var OurSet = SortedSet[Name] _
    override def variable = {
      _ match {
        case name => Variable(name)//SortedSet+(name).andThen(Variable(_))
      }
    }
}

Is there syntax to chain like a void function that adds to the SortedSet and then wait? I can’t use andThen because I want to do two things, I want to add it to the Set and then I want to return the variable. Any ideas?

Answer 1

I think you mean something like this:

var ourSet = Set[String]()
def func(s: String) = 
  s match {
    case name =>        // a `case` can be followed by multiple statements 
      ourSet += name    // first we add `name` to the set
      name              // the last expression gets passed up to the assignment of x
  }
val x = func("test")
// ourSet is now Set("test")
// x is now "test"

A match expression will evaluate to the last expression of the matching case. Here, the case that matches is case name. The last expression under the case name block is name, so that’s what the entire match evaluates to. So the function func returns name, which is "test" when we call func("test"). Thus,xis assigned to be“test”`.

In addition, you can perform any other operations inside the case block you want. Here, we are modifying ourSet.