How to check lambda emptiness in kotlin

Question

How can I check if lambda is empty in Kotlin? For example, I have signature like

onError:(Throwable) -> Unit = {}

How can I differ is its default value comes to body or value applied to this function?

Answer 1

You couldn't test if the body of an lambda is empty (so it contains no source-code) but you can check if the lambda is your default value by creating a constant for that value and use this as default value. Than you can also check if the value is the default value:

fun main(args: Array<String>) {
    foo()
    foo { }
    foo { println("Bar") }
}

private val EMPTY: (Throwable) -> Unit = {}
fun foo(onError: (Throwable) -> Unit = EMPTY) {
    if (onError === EMPTY) {
       // the default value is used
    } else {
       // a lambda was defined - no default value used
    }
}

Answer 2

Just don't use empty lambdas as defaults.

Since in kotlin nullability is a first class citizen and the clear indication of a missing value, I would pass null as the default argument.

onError: ((Throwable) -> Unit)? = null

The signature looks a bit ugly, but I think it's worth it.

For example it can be easily checked and handled and you don't expect that the lambda does something useful - especially for lambdas with a return value.

I gave the same comment here: https://stackoverflow.com/a/38793639/3917943