Kotlin run、let、apply、with

@kotlin.internal.InlineOnly
public inline fun  run(block: () -> R): R {
    contract {
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    }
    return block()
}

@kotlin.internal.InlineOnly
public inline fun  T.let(block: (T) -> R): R {
    contract {
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    }
    return block(this)
}

@kotlin.internal.InlineOnly
public inline fun  T.apply(block: T.() -> Unit): T {
    contract {
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    }
    block()
    return this
}

@kotlin.internal.InlineOnly
public inline fun  with(receiver: T, block: T.() -> R): R {
    contract {
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    }
    return receiver.block()
}

@kotlin.internal.InlineOnly
@SinceKotlin("1.1")
public inline fun  T.also(block: (T) -> Unit): T {
    contract {
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    }
    block(this)
    return this
}

源码点进去就能看到Standard.kt类中。许多扩展函数像list的First（）、takeLast（）、takeLastWhile（）、reversed（）forEach（），String的isNotBlank()、iterator（）、takeIf（）判断条件等。举例就像forEach（），我们点进去看看

 发现里面For循环 OO