Be careful with Kotlin Scope Functions

The Curious Case of ?.let {} ?: run {}

Try this code :

fun main() {
    val theCase: Int? = 1
    theCase?.let {
        fun1()
        fun2()
    } ?: run {
        funError()
    }
}

fun fun1() {
    println("fun1")
}

fun fun2(): Unit? {
    //    return println("fun2")
    return   null
}

fun funError(){
    println("funError")
}

The result :

fun1
funError

So the funError() was called - but it shouldn’t?

Play with the code and change the fun2() to :

fun fun2(): Unit? {
    return println("fun2")
//    return   null
}

Generally speaking, funError() should be called only when (?)

val theCase: Int? = null

Not really …

You can read more about it (and many others) in details here :

Effective Kotlin