let

Learn null-safe blocks with let.

let for a null-safe block

The ?.let { } combo runs the lambda when the receiver is not null; it does nothing when it is null. Inside the lambda the receiver is known as it and has the non-null type.

val name: String? = "Ava"
name?.let {
    println("Hello $it")
}
// Hello Ava

This form says "if not null, do a few things"; it is a short alternative to the if (x != null) block.

it and chained let

Inside ?.let the receiver becomes it automatically, with its non-null type. Multiple let calls can be chained, but nested it hurts readability; an explicit parameter name ({ value -> ... }) is then better.

name?.let { value ->
    println("Hello $value")
}

Here the name value replaces it; it makes the code more readable.

Exercise Greet with let in a null-safe way

Define a val name: String? = "Ava". Print a null-safe greeting with name?.let { println("Hello $it") }.

Show example solution

fun main() {
    val name: String? = "Ava"
    name?.let { println("Hello $it") }
}

Test yourself

Fill in the blank: let block

name?.____ { println(it) }

Show answer

name?.let { println(it) }

Match expression to role

?.let { }

Run when not null
it

Single lambda parameter
name?.length

Returns null if null
name ?: "None"

Default if null
name?.let { println(it) } — what happens if name is null?
- Throws an exception
- Prints println(null)
- Lambda does not run, returns null
- it becomes null
Show answer

Lambda does not run, returns null — ?.let does not run the lambda on a null receiver; it returns null.

Next lesson: Null check

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