Kotlin 函数声明：大括号前的等号

Despite visual similarity, the idea of these two declarations is completely different.

1.不带等号的函数声明

没有等号的函数声明是Unit-returning function(类似于Java的void个函数).

花括号内是它的主体，在函数调用时立即执行.可以使用明确指定的Unit重写函数:

fun foo1(): Unit {
    println("baz1")
    return Unit
}

Kotlin不需要Unit个返回函数的return语句和显式返回类型，而且通常都会被省略.

2.带等号的函数声明

带等号的函数声明是single-expression function，它所做的就是返回等号右边的内容.

一个更简单的例子:fun getInt() = 1只是fun getInt(): Int { return 1 }的缩写形式.

在foo2中，右手边是a lambda.lambda代码块中的代码是not executed.换句话说，foo2是一个返回另一个函数的函数.

Return type of foo2 is () -> Unit, a function itself, and thus foo2 is a higher-order function.

没有语法糖和显式类型，foo2可以重写为

fun foo2(): () -> Unit {
    val result: () -> Unit = { println("baz2") }
    return result
}

As to the usage, the function which foo2 returns can be stored in a variable, passed around and can later be invoked:

val f = foo2()
f() //equivalent to
f.invoke()

This is also why foo2()() in the example executes the code from the lambda body.

Alternatively, we can add () at the end when we declare foo2(), as shown in the following example. As such, the lambda expression will be invoked when calling foo3(). But this is not a good pattern.

fun foo3() = {
    println("baz3")
}()