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When working with lambdas, whenever we want to execute the code inside their body, we need to call the invoke() function on them or its equivalent, the () operator:

val callback: () -> Unit = { println("The job is done!") }
callback.invoke()
callback()

The preceding code is going to print the text twice:

"The job is done!"
"The job is done!"

There is also another clean way of passing functions as the parameters to other functions. We can do it using function references:

fun hideView(view: View): Unit  {
    view.visibility = View.INVISIBLE
}

submitButton.setOnClickListener(::hideView)

The function references approach can be particularly useful for reusing the function implementation across the codebase.