Pattern for optional-parameters in Scala using null - scala

Question

I have a function which takes optional parameters. However, these are not Option[?] but can be either set or null:
private def div(id: String = null, cssClass: String = null): JQuery = {
val optId = Option(id)
val optCssClass = Option(cssClass)
...
// deal with optId and optCssClass using the Scala-way™
...
}
I am using "null", which I know should be avoided like the plague. However, it allows me to write code like this:
div(id = "someId") // no cssClass
div(id = "otherId", cssClass = "someClass")
which to my eyes looks nicer than:
div(id = Some("someId")) // no cssClass
div(id = Some("otherId"), cssClass = Some("someClass"))
Is this a known / acceptable Scala pattern? (using null as default parameter values and converting to Option)
Or is it still heressy / bad-practice? If so, why?

Answer 1

why not replace null with empty string ?
private def div(id: String = "", cssClass: String = ""): JQuery = {
val optId = if(id.isEmpty) None else Some(id)
val optCssClass = if(cssClass.isEmpty) None else Some(cssClass)
...
// deal with optId and optCssClass using the Scala-way™
...
}
then you can do:
div(id = "someId") // no cssClass
div(id = "otherId", cssClass = "someClass")

Answer 2

One more approach I can suggest is a Builder Pattern
trait ElementBuilder {
def identified(id: String): ElementBuilder
def build: JQuery
}
case class DivElement(identifier: Option[String] = None)
extends ElementBuilder {
def identified(id: String) = this.copy(identifier = Option(id))
def build: JQuery = ??? // Smth like <div id={identifier}></div>
}
val builder = DivElement()
builder.identified("foo")
val element = builder.build
This approach allows you explicitly set parameters and then build you element by them

Answer 3

Most of the answers here are proposing some variant of the "null object" pattern, by denoting an empty String to mean "undefined" (as in val optId = if(id.isEmpty) None else Some(id))
The catch here is that an empty string might be a valid value! This is true of any String, though you can mitigate the problem by using something really outrageous, possibly involving non-printable characters. e.g:
val UndefinedString = "THIS-IS-A-REALLY-UNLIKELY-VALID-VALUE"
private def div(
id: String = UndefinedString,
cssClass: String = UndefinedString
): JQuery = {
val optId = Option(id) filter (_ != UndefinedString )
val optCssClass = Option(cssClass) filter (_ != UndefinedString )
...
// deal with optId and optCssClass using the Scala-way™
...
}
Better still, you could use a different type to denote your null object. As you can't subclass String you'll have to bump your params up the type hierarchy and make them CharSequences
object NullCharSeq extends CharSequence {
def charAt(idx: Int): Char = ???
def length(): Int = 0
def subSequence(start: Int, end: Int): CharSequence = this
def toString(): String = ???
}
def charSeqToOptStr(cs: CharSequence): Option[String] = cs match {
case NullCharSeq => None
case x => Option(x) map (_.toString)
}
private def div(
id: CharSequence = NullCharSeq,
cssClass: CharSequence = NullCharSeq
): JQuery = {
val optId = charSeqToOptStr(id)
val optCssClass = charSeqToOptStr(cssClass)
...
// deal with optId and optCssClass using the Scala-way™
...
}
It's a heavyweight pattern for one-shot usage, but the cost is quickly amortized if you use it a lot (NullCharSeq and charSeqToOptStr only need to be defined once in the codebase).
There's also zero risk of mistakenly passing your "undefined" String as though it were a valid value. Plus, you gain the ability to directly accept CharBuffer/StringBuffer/StringBuilder as your arguments.

Answer 4

I would also go for a special string like the empty string in #Jiafeng's answer, if this is a sensible value. You could also define a string, like
val NoId = "?"
def div(id: String = NoId) = id match {
case NoId => None
case x => Some(x)
}
Another approach would be to use another type which can be implicitly created from string or absence.
sealed trait MaybeId
implicit class Id(val name: String) extends MaybeId
case object NoId extends MaybeId
def div(id: MaybeId = NoId) = id match {
case NoId => None
case x: Id => Some(x.name)
}
Here is a general type that behaves like Option[A] with implicit conversion A => Some[A].