Why does passing a variable to a lambda in Kotlin works?

Question

Suppose I have high order function that accepts a lambda as a parameter like this:

fun getNum(op: () -> Int) = op()

And a function that returns a number:

fun getTen() = 10

In the main function I can call the getNum() function like this

fun main(args: Array<String>){
    val x = 50
    val a = getNum(::getTen)    // a == 10
    val b = getNum{x}           // this works and b == 50
}

Why does passing a varibale instead of lambda works? Any idea? Thanks.

hotkey · Accepted Answer · 2018-08-27 22:49:16Z

3

In Kotlin, the last or the single expression in a lambda is the return value.

In the getNum { x } expression, { x } is a lambda with a single expression x in it, which is thus considered the return value, so when the lambda is invoked, it only evaluates x captured in the closure and returns its value back to getNum.

The getNum(::getTen) call is, in turn, the usage of a callable reference (it is distinguishable by the :: sign), which can be passed as a value of the functional type () -> Int just the same as if it would be getNum { getTen() }.

See: Lambda Expressions and Anonymous Functions in the language reference.

answered Aug 27, 2018 at 22:49

hotkey

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