Looking for a more idiomatic way to do conditional logging during a list map

Question

There's gotta be a more Kotlin-esque and terse way to do this in Kotlin, but it's not coming to me. Imagine you're doing a mapNotNull operation. Items which cannot be mapped are converted to null to be filtered out. Items that cannot be mapped also result in a warning being printed. This code works, but it's really verbose. Can you help me trim it down?

    val listOfStrings = listOf("1","2","3","not an int", "4","5")

    val convertedToInts = listOfStrings.mapNotNull {
        val converted = it.toIntOrNull()
        if(converted == null){
            println("warning, cannot convert '$it' to an int")
        }
        converted
    }

Answer 1

I think your code is idiomatic and readable as it is. I prefer to write it with the explicit null-check.

But if you really want to make a shorter one-liner, you could do something like below. But it looks very hacky with the null.apply {} which is needed to return null instead of Unit from the right side of the elvis-operator:

val listOfStrings = listOf("1","2","3","not an int", "4","5")

val convertedToInts: List<Int> = listOfStrings.mapNotNull {
    it.toIntOrNull() 
        ?: null.apply { println("warning, cannot convert '$it' to an int") }
}

You could also use run which looks a bit more readable:

?: run { 
    println("warning, cannot convert '$it' to an int")
    null
}