I might me missing something obvious here but I'm trying to introduce nullable function parameter like

fun <T : Any?> execute(queryId: (() -> UUID)? = null, operation: Operation<T>): T? {

but I can't invoke this by

execute(Operation(...))

only

execute(null, Operation(...))

works, any pointers?

You can only leave out trailing optional parameters, not leading ones. If you just want to pass the

operation

parameter you need to use named-parameters when calling:

execute(operation=Operation(...))

Ah, thats correct. Thanks.

so following this language rule you could move the lambda to be the last parameter, then you could call the function like this:

execute(Operation(...))

or

execute(Operation(...)) {
    // new uuid
}