Steps

1. How to define a function with a callback parameter

Let's say that you have a function named printReport() which needs to be passed in a function for sending email once the report is completed.

You can define the printReport() function to have a call-by-name parameter of type unit that will essentially be the email callback function.

println("Step 1: How to define a function with a callback parameter")
def printReport(sendEmailCallback: () => Unit) {
  println("Printing report ... started")
  // look up some data in database and create a report
  println("Printing report ... finished")
  sendEmailCallback()
}

2. How to call a function which has a callback parameter

When calling the printReport() function from Step 1, you will need to pass in a callback function for sending email.

println("\nStep 2: How to call a function which has a callback parameter")
printReport(() =>
println("Sending email ... finished")
)

You should see the following output when you run your Scala application in IntelliJ:

Step 2: How to call a function which has a callback parameter
Printing report ... started
Printing report ... finished
Sending email ... finished

3. How to call a function without providing its callback parameter

What if you wanted to print the report but not send any email?

println("\nStep 3: How to call a function without providing its callback parameter")
// printReport() // You get compile time error

NOTE:

• Calling printReport() function without passing a callback function will give you a compile time error.

You can instead pass in an empty anonymous function as shown below, although the syntax does not look that elegant:

printReport(() => {}) // Not that elegant.

4. How to define a function with an Option callback

Let's make use of what you've learned from the Option Tutorial and define another printReport() function which would have an Option callback parameter.

println("\nStep 4: How to define a function Function with an Option callback")
def printReportWithOptionCallback(sendEmailCallback: Option[() => Unit] = None) {
  println("Printing report ... started")
  // look up some data in database and create a report
  println("Printing report ... finished")
  sendEmailCallback.map(callback => callback())
}

NOTE:

• We are using the map() function to filter out any None callback function.

5. How to call a function without providing its callback parameter

With the printReportWithOptionCallback() function from Step 4 above, you can now more elegantly call the print report function without having to specify any callback function.

println("\nStep 5: How to call a function without providing its callback parameter")
printReportWithOptionCallback() // more elegant

You should see the following output when you run your Scala application in IntelliJ:

Step 5: How to call a function without providing its callback parameter
Printing report ... started
Printing report ... finished

6. How to call a function with Option callback parameter

If you've followed the tutorial on Option, the syntax for calling a function with an Option parameter should be familiar to you.

With regards to the callback function parameter, you now need to wrap it within Some()

println("\nStep 6: How to call a function with Option callback parameter")
printReportWithOptionCallback(Some(() =>
  println("Sending email wrapped in Some() ... finished")
))

You should see the following output when you run your Scala application in IntelliJ:

Step 6: How to call a function with Option callback parameter
Printing report ... started
Printing report ... finished
Sending email wrapped in Some() ... finished