Question

Is it possible to use named arguments in a Scala constructor, and later on override getters and setters without breaking the constructor interface or making the code extremely ugly?

Take the following bit of scala code

class Person( var FirstName: String, var LastName: String )

Nice and clean. This would create a simple class called person, which we could use in the following way

val john = new Person( FirstName="John", LastName="Doe" )
john.FirstName = "Joe"
println( john.FirstName )

Later, we decide we want to add some validation to the FirstName setter. As such, we create a new private local variable and override the getter and setter methods

class Person( var _FirstName: String, var _LastName: String ) {

    def FirstName = _FirstName  
    def FirstName_= (value:String) = _FirstName = value

}

Still somewhat clean, however in order to do this, we’ve had to change the constructor argument names, thus breaking the external interface.

The first solution to this problem I came up with was

class Person {
    var _FirstName:String = null 
    var LastName:String  = null

    def FirstName = _FirstName  
    def FirstName_= (value:String) = _FirstName = value

    def this( FirstName: String, LastName: String ){
        this()
        this._FirstName = FirstName
        this.LastName = LastName 
    }

}

Which is somewhat ugly and inelegant, and removes most of the nice reasons I was using scala in the first place.

Is there a better way of doing this?

tl;dr How to override getters/setters for members defined in the default constructor without making the code ugly or changing the public interface?

Answer 1

If you’re not already using implicit conversions to create the arguments, you can do something like this:

def validateName(s: String) = {
  if (s.length>0 && s(0).isUpper) s
  else throw new IllegalArgumentException(s+" is not a name!")
}

object Example {
  private[Example] class ValidatedName(val s: String) { }
  class Person(var firstName: ValidatedName, var lastName: String) { }
  implicit def string2valid(s: String) = new ValidatedName(validateName(s))
  implicit def valid2string(v: ValidatedName) = v.s
}

scala> new Example.Person("Joe","Schmoe")
res17: Example.Person = Example$Person@51887dd5

scala> new Example.Person("ee","cummings")
java.lang.IllegalArgumentException: ee is not a name!

It’s not binary compatible, but it is source compatible (again, if the names weren’t already relying upon implicit conversions).

Another slightly longer possibility is to create a stealth ancestor:

class CheckedPerson(private var first: String, var lastName: String) {
  def firstName = first
  def firstName_=(s: String) { first = validateName(s) }
}
class Person(firstName: String, lastName: String) extends
  CheckedPerson(validateName(firstName),lastName) { }

for which I’m not sure about binary compatibility, but will definitely give source compatibility.