Scope Functions

The standard library provides five scope functions — let, run, with, apply, and also — that execute a block in the context of an object. They differ in two ways: how the object is referenced (it vs this), and what the block returns.

The cheat sheet

Function	Object reference	Returns	Typical use
`let`	`it`	block result	null-checks, transform
`run`	`this`	block result	configure + compute a result
`with`	`this`	block result	group calls on an object
`apply`	`this`	the object	configure and return it
`also`	`it`	the object	side effects (logging)

`let`

Transform a value, or run code only when non-null:

val length = "hello".let { it.length }     // 5

nickname?.let {
    println("Nickname: $it")               // runs only if non-null
}

`run`

Like let, but the receiver is this. Good for configuring then returning a result:

val area = rectangle.run { width * height }

`with`

Group several calls on the same object (not an extension):

val summary = with(user) {
    "$name is $age years old"
}

`apply`

Configure an object and return the object itself — great for builders:

val file = StringBuilder().apply {
    append("line 1\n")
    append("line 2\n")
}.toString()

`also`

Perform a side effect and return the object unchanged:

val nums = mutableListOf(1, 2)
    .also { println("before: $it") }
    .apply { add(3) }
    .also { println("after: $it") }

Choosing one

Return a result? → let (uses it) or run (uses this).
Return the object after configuring it? → apply (uses this).
Return the object after a side effect? → also (uses it).

Do not overuse them — a plain local variable is often clearer than a nested chain of scope functions.

Exercises

Use apply to create a configured StringBuilder and return its string.
Write formatUser(name: String?, age: Int): String that returns "name (age)", or "unknown (age)" when name is null — using let and the Elvis operator.

Solution

   fun formatUser(name: String?, age: Int): String =
       (name?.let { it } ?: "unknown") + " ($age)"

</details>

This solution is in examples/core/part5/ and is tested by CI.

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