What is Trailing Lambda and Comma in Kotlin?

Trailing lambda is something new in Kotlin that other programming language doesn't have. At least, I do not aware of any other programming language that supports it.

When you see code like this:

var result = operateOnNumbers(a, b) { input1, input2 ->
    input1 + input2
}
println(result)

It means operateOnNumbers() has 3 parameters. The last parameter is a function definition, which you usually either pass in the function reference or lambda.

var result = operateOnNumbers(
    input1 = a,
    input2 = b,
    operation = { input1, input2 ->
        input1 + input2
    }
)
println(result)

Somehow I am still not getting used to this trailing lambda syntax. It looks like a function implementation.

So my mind always needs to automatically map to this (the code outside the parentheses is the last parameter of the function) every time I see the Trailing Lambda syntax.

The signature and implementation of operateOnNumbers() looks like this:

fun operateOnNumbers(
    input1: Int,
    input2: Int,
    operation: (Int, Int) -> Int): Int {

    return operation(input1, input2)
}

On the other hand, trailing commas is pretty common in other languages.

With Trailing Comma

var result = operateOnNumbers(
    a, 
    b, // trailing comma here
) { input1, input2 ->
    input1 + input2
}

Without Trailing Comma

var result = operateOnNumbers(
    a, 
    b // no trailing comma here
) { input1, input2 ->
    input1 + input2
}

The benefit of using it is allowing easier diff and merge. For me, it makes my copy-and-paste life easier. Yupe, I do a lot of copy & paste!

Conclusion

I hope you enjoy this short post. I want to blog about this (especially Trailing Lambda) because it sometimes looks confusing to me. The function call is a bit complex. I always need to remind myself, the code outside the parentheses is the last parameter of the function.