Understanding let, apply, with, run in Kotlin

Issue #114

Picture worths thousand words. Code worths thousand pictures. I don’t understand much until I take a look at Standard.kt in Kotlin standard library.

/**
 * Calls the specified function [block] with `this` value as its receiver and returns `this` value.
 */
@kotlin.internal.InlineOnly
public inline fun <T> T.apply(block: T.() -> Unit): T {
    contract {
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    }
    block()
    return this
}

/**
 * Calls the specified function [block] with `this` value as its argument and returns its result.
 */
@kotlin.internal.InlineOnly
public inline fun <T, R> T.let(block: (T) -> R): R {
    contract {
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    }
    return block(this)
}

• apply returns T and accepts a block of type T.() -> Unit. This is called Function Literals with Receiver

Kotlin provides the ability to call a function literal with a specified receiver object. Inside the body of the function literal, you can call methods on that receiver object without any additional qualifiers. This is similar to extension functions, which allow you to access members of the receiver object inside the body of the function

• let returns R and accepts a block of type T -> R. Inside the block, you can access it which is it: implicit name of a single parameter