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Kotlin provides 5 scope functions that take a receiver and a lambda. They differ in (a) what this refers to, and (b) what the function returns.

let — usually for null checks + transformations:

val len: Int? = name?.let { it.length }    // null-safe transform

apply — configure an object then return it:

val user = User().apply {
    name = "Ada"
    email = "[email protected]"
}

also — side effects, return the original:

val user = createUser().also { logger.info("created $it") }

run — like apply but returns the lambda result:

val result = transaction.run {
    debit(100)
    credit(50)
    commit()             // last expr — returns whatever commit() returns
}

with — like run but takes the object as a parameter (different syntax):

val summary = with(user) {
    "$name ($email): $age"
}

Mnemonic: apply/also return the receiver (chainable). let/run/with return the lambda result.