匿名函数：常规调用 VS 速记语法

Question

I'm new to Kotlin.

I was experimenting with Anonymous functions a little bit until facing different outputs for the same concept using different approaches.

• First: Creating my Anonymous function:

var greetingFunction = { playerName: String , numBuildings: Int ->
    val currentYear = 2022
    println("Adding $numBuildings houses")
    "Welcome to SimVillage, $playerName! (Copyright $currentYear)\n"
}

• Second: Creating a function that takes another function as a parameter:

private fun runSimulation(playerName: String, greetingFunc: (String, Int) -> String){
    val numOfBuildings = (1..3).shuffled().last()
    println(greetingFunc(playerName, numOfBuildings))
}

A- Regular call for Anonymous function:

println(runSimulation("Ahmed", greetingFunction))

Output:

Adding 3 houses
Welcome to SimVillage, Ahmed! (Copyright 2022)

---

B- Shorthand call for Anonymous function:

println(runSimulation("Different") { playerName: String , numBuildings: Int ->
    val currentYear = 2022
    println("Adding $numBuildings houses")
    "Welcome to SimVillage, $playerName! (Copyright $currentYear)\n"
})

Output:

kotlin.Unit
Adding 2 houses
Welcome to SimVillage, Different! (Copyright 2022)

kotlin.Unit

I tried to remove the println() and calling the runSimulation function directly and the output was:

Output:

kotlin.Unit
Adding 2 houses
Welcome to SimVillage, Different! (Copyright 2022)

What I really want to know is: how in the first place did I get that "kotlin.Unit" print using the Shorthand Syntax?

Answer 1

Kotlin will automatically infer the type of anonymous functions . Since the last line of greetingFunction is a String

var greetingFunction = { playerName: String , numBuildings: Int ->
    val currentYear = 2022
    println("Adding $numBuildings houses")

    // the last line is a string -> Kotlin will infer that the return type is a String
    "Welcome to SimVillage, $playerName! (Copyright $currentYear)\n"
}

the inferred type is

var greetingFunction: (String, String) -> String

Return types for block-body functions are not inferred . If a function does not return a useful value, its return type is Unit . The function

private fun runSimulation(playerName: String, greetingFunc: (String, Int) -> String) {
    val numOfBuildings = (1..3).shuffled().last()
    println(greetingFunc(playerName, numOfBuildings))
}

will therefore return Unit , so

println(runSimulation("foo") { _, _ -> "bar" })

will print the returned value of runSimulation() , and Unit.toString() is kotlin.Unit

Since runSimulation() will also print to stdout, this is effectively the same as running

println(println("bar"))

First the 'inner' println() will output bar , and then the 'outer' println() will print kotlin.Unit .

bar
kotlin.Unit