Kotlin: Error "change lambda expression return type to Unit" in setOnNavigationItemSelectedListener

I am developing an app in Kotlin and using a BottomNavigationView to navigate between different fragments via a ViewPager. I am trying to implement the setOnNavigationItemSelectedListener method to change the ViewPager page when a navigation item is selected.

However, I encounter an error: "change lambda expression return type to Unit". I am not sure why this error occurs because similar code examples I found online do not have this issue. Here is a screenshot of the error:

Interestingly, when I use setOnNavigationItemReselectedListener instead of setOnNavigationItemSelectedListener, the error goes away, but I need the setOnNavigationItemSelectedListener to work for item selection, not reselection.

Questions:

1. Why am I getting the "change lambda expression return type to Unit" error with setOnNavigationItemSelectedListener?

2. How can I resolve this error and properly implement the listener?

Any help would be greatly appreciated!

Add true in your lambda, right after the when statement. This functional interface requires you to return a Boolean value. The last expression of a lambda is the return value.

For less repetitive code, you may consider modifying it as shown:

bottomNavigationView.setOnNavigationItemSelectedListener {
    val page = when (it.itemId) {
        R.id.navigator_main -> 0
        R.id.navigator_search_food -> 1
        R.id.navigator_list_of_week -> 2
        R.id.navigator_for_students -> 3
        else -> -1
    }
    viewPager.setCurrentItem(page)
    true
}