How to pass an optional closure parameter to a function?

Question

I would like to pass a closure to a method as an Option and I am doing what is shown below. I get a compile error as shown below. Is it possible to pass an optional closure parameter to a function?

def sampleMethod(a: String, b: String, optionalMethod: Option[(String, Int) => Unit]) {
    // do some processing with a and b
    optionalMethod match {
      case Some(optionalMethod) => {
        optionalMethod("a",3)
      }
      case _
      log("no optional method passed")
    }
}

// definition of optMethod in some other place
val optMethod = (c: String, d: Int) => {
  // some processing with c, d and external values 
}

// invoke
sampleMethod("hi", "bye", optMethod) => FAILS TO COMPILE

ERROR = type mismatch. expecting Option[(String, Int) => Unit] found (String, Int) => Unit

Answer 1

The error message is pretty explicit: sampleMethod expects an Option, but you are passing a straight function value (not wrapped in Some).

The simplest way to fix this is to wrap optMethod into a Some:

sampleMethod("hi", "bye", Some(optMethod))

But if you want to be able to simply do sampleMethod("hi", "bye", optMethod), you could add overloaded definitions of sampleMethod:

object Test {
  def sampleMethod(a: String, b: String, optionalMethod: Option[(String, Int) => Unit]) {
    // do some processing with a and b
    optionalMethod match {
      case Some(optionalMethod) => {
        optionalMethod("a",3)
      }
      case _ => log("no optional method passed")
    }
  }
  def sampleMethod(a: String, b: String) { sampleMethod(a, b, None) }
  def sampleMethod(a: String, b: String, optionalMethod: (String, Int) => Unit) {
    sampleMethod(a, b, Some(optionalMethod)) 
  }
}

val optMethod = (c: String, d: Int) => {
  // some processing with c, d and external values 
}

// invoke
Test.sampleMethod("hi", "bye", optMethod) // Now Compiles fine
Test.sampleMethod("hi", "bye") // This too

Answer 2

As pointed out earlier, your method expects an Option value containing the optionalMethod. You must hence pass an Option value to it:

// invoke with method
sampleMethod("hi", "bye", Some(optMethod))
// invoke without method
sampleMethod("hi", "bye", None)

If you want to avoid the Option value (esp. avoid the None), you can try the following:

def sampleMethod(a: String, b: String, optionalMethod: (String, Int) => Unit = (_, _) => log("no optional method passed")) {
  optionalMethod("a", 3)
}

// invoke with method
sampleMethod("hi", "bye", optMethod)
// invoke without method
sampleMethod("hi", "bye")

Answer 3

How about

sampleMethod("hi", "bye", Some(optMethod))