Kotlin: naming parameters in lambda

Question

Please ignore the actual "functionality" and concentrate more on the use of lambda here as I am hacking around with lambdas, let, also, run etc to get a feeling for Kotlin.

val listener : (String?)->String = {
            val s2 = it?.also {
            }
                ?: "Null"
            statusText.text=s2
            s2
        }

So this assignment of a lambda to "listener" is just fine.

Could someone tell me why I am unable to assign a name to the first (and only) parameter eg.

  val listener : (s: String?)->String = {
            val s2 = s?.also {
            }
                ?: "Null"
            statusText.text=s2
            s2
        }

In the line "val s2=s?.also..." the compiler complains that "s" is an unresolved reference. If so why is the naming of the parameter legal eg:

val listener : (s: String?)->String = {

Any explanation would be a great help to my understanding.

JB Nizet · Accepted Answer · 2019-06-20 06:14:45Z

4

It should be

val listener : (s: String?) -> String = { s ->
    val s2 = s?.also {
    } ?: "Null"
    statusText.text=s2
    s2
}

or simply

val listener : (String?) -> String = { s ->
    val s2 = s?.also {
    } ?: "Null"
    statusText.text=s2
    s2
}

Note that using return in the mambda is incorrect, too.

answered Jun 20, 2019 at 6:14

JB Nizet

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10 Comments

RichieHH Over a year ago

Yeah cut and paste hell re:return. I altered that before I saw your reply. Thanks. Though I do see there is an explicit way to use return in lambdas eg return @listener s2, though I am yet to try that. I'll mark your lesson as accepted. Thank you.

Slaw Over a year ago

What sort of situations would we want to name the parameter then (i.e. is there any benefit)?

RichieHH Over a year ago

When I want to use a named parameter: when the lambda takes more than one. Is this something frowned upon? That aside I made my point that this is just experimenting with Kotlin to get a feeling - not specifically to solve an issue.

JB Nizet Over a year ago

When a function takes a function as argument for example, or returns a function. Naming the arguments of the argument/returned function makes it easier to understand what the function does. See kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/… for example: naming the Int argument index makes it easier to unerstand what the argument represents.

RichieHH Over a year ago

I'm not questioning you. I am referring to Slaw. But naming parameters is frequently done and my Q was merely on how to do it properly: and you have answered. Thanks,

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