How to omit lambda parameters in Kotlin?

Question

Sometimes I don't use all lambda parameters, like the second parameter in the example below, which is an old value in a change event:

selected.onChange { value, _ ->
    checkBox.isChecked = value
}

How do I omit them? Cause that clutters the code and hurts readability. Previously I used the method above, but in kotlin 1.0.0-beta-1103 underscores are reserved.

Answer 1

As of 1.1, you can do exactly that:

Underscore for unused variables (since 1.1)

If the lambda parameter is unused, you can place an underscore instead of its name:

map.forEach { _, value -> println("$value!") }

https://kotlinlang.org/docs/reference/lambdas.html#underscore-for-unused-variables-since-11

I should add that the compiler now generates a warning for unused lambda parameters, and there's a new corresponding quick fix for Android Studio

Answer 2

You can use escaping for Java identifiers that are keywords in Kotlin (i.e. backticks) or overload onChange.

Backticks

selected.onChange { value, `_` ->
    checkBox.isChecked = value
}

Overload onChange

interface Listener<T1, T2> {
    fun onChange(f: (T1, T2) -> Unit) // original method
    fun onChange(f: (T1) -> Unit)     // new method
}

And if you cannot change Listener<T1, T2> (or whatever type settable is from your example) then you can use an extension function:

fun <T1, T2> Listener<T1, T2>.onChange(f: (T1) -> Unit) = onChange { t1, t2 -> f(t1) }

You can then call onChange like you want:

selected.onChange { value ->
    checkBox.isChecked = value
}