How to understand the fun buildString(builderAction: (StringBuilder) -> Unit) : String in Kotlin?

Question

The following code is from https://github.com/gbaldeck/learning-kotlin/blob/master/src/main/kotlin/org/learning/DSLconstruction.kt

I find it hard to understand.

1: The fun buildString only accept one lambda parameter in Section A, why are there two parameters passed in Section B?

2: What is full code of Section B?
Such as

 val s = buildString { aa : StringBuild -> aa.append("Hello.") } // I don't know whether it's right? 

3: What is this it in Section B? Does this it represent StringBuild ?

Section A

fun buildString(builderAction: (StringBuilder) -> Unit ) : String {
    val sb = StringBuilder()
    builderAction(sb)
    return sb.toString()
}

Section B

val s = buildString {
    it.append("Hello, ")
    it.append("World!")
}

logError(s)  //The result is Hello, World!

Answer 1

Yes, it is the StringBuilder . It is named it by default. You can specify the name if you want to.

buildString {
     it.append("...")
}

is the same as

buildString { it ->
    it.append("...")
}

or even

buildString { myNewString -> 
    myNewString.append("...")
}

Answer 2

1. There is only one parameter being passed in section B, namely, this parameter:

    { it.append("Hello, ") it.append("World!") } 

   That is one lambda expression, not two. The lambda expression has 2 lines , but it's still one lambda.

2. If you want to expand the call to buildString ,

    val builder = StringBuilder() builder.append("Hello, ") builder.append("World!") val s = builder.toString() 

3. Yes, the it refers to the StringBuilder sb in buildString . When the function type has only one parameter, you can refer to the single parameter with it in the lambda expression without giving it a name.

Answer 3

1: The fun buildString only accept one lambda parameter in Section A, why are there two parameters passed in Section B?

There is only 1 parameter passed to that function: specifically, the builderAction of type (StringBuilder) -> Unit .

So

val s = buildString {
    it.append("Hello, ")
    it.append("World!")
}

is equivalent to

val s: String = buildString(builderAction = { stringBuilder: StringBuilder ->
    stringBuilder.append("Hello, ")
    stringBuilder.append("World!")
    // return Unit
})

Meaning it is actually the unnamed single argument of (StringBuilder) -> Unit , so it's a StringBuilder .