Question

Suppose I’m writing a GUI

class Kitteh (val age: Int) {
  require (age < 5)
  def saveMeow(file: File) = { /* implementation */ }
  def savePurr(file: File) = { /* implementation */ }
}

The frame has a field for the current Kitteh, which is an Option because it might not have been defined yet, or the user may have attempted to create an invalid one:

var currentKitteh: Option[Kitteh] = None

Now I want to create a Kitteh safely when the user hits Create

val a = ... // parse age from text box
currentKitteh = try { Some(new Kitteh(a)) } catch { case _ => None }

My GUI has two buttons which do similar things. In psedocode, they should both

if (currentKitteh.isDefined) {
  if (file on the disk already exists) {
    bring up a dialog box asking for confirmation
    if (user confirms)
       << execute method on currentKitteh >>
  }
}
else bring up warning dialog

Don’t worry about the detail: the point is that because there is code duplication, I want to create a common method that I can call from both buttons. The only difference is the method on the Kitteh that needs to be executed.

Now if currentKitteh were not an Option, the common method could have a signature like

def save(filename: String, f:(File => Unit)) {

which I could call with, for example

save("meow.txt", currentKitteh.saveMeow _)

but since it is actually an Option, how could I implement this?

I could just check whether currentKitteh is defined, and do a .get before calling the save method for each button, but is there another way, leaving this check in the save method? In other words, given an Option[A], is it possible to specify a partial function from a method on the (possibly non-existent) A object?

(hope this question makes sense, convoluted example notwithstanding)

edit: Bonus question: what if, instead of Option[Kitteh], I used Either[Throwable, Kitteh]?

update: Additional line added to pseudocode to bring up warning dialog: ideally, the save method should always be called so that the user is warned if there is no valid Kitteh to save.

Answer 1

This looks like the best option to me:

currentKitteh foreach { c => save("meow.txt", c.saveMeow _) }

If you’re repeatedly doing this, you can abstract it,

def currentSaveMeow(file: String) = currentKitteh foreach { c =>
  save(file, c.saveMeow _)
}
currentSaveMeow("meow.txt")

I suppose to answer your original question, you could also push the logic into the function argument,

save("meow.txt", file => currentKitten.foreach(_.saveMeow(file)))

The semantics are a little different with this version.

Update. If k: Option[Kitteh] is replaced by k: Either[Throwable, Kitteh], then what about k.right foreach { c => ... }? You could also use k.right map ... if you want to preserve error information.

---

In response to the modified question, here’s another abstraction possibility,

def save(filename: String, f: (Kitteh, File) => Unit)

Now save has the responsibility of unpacking currentKitteh. Call save like this,

save("meow.txt", (k, f) => k.saveMeow(f))

or like this,

save("meow.txt", _ saveMeow _)