[SOLVED] Kotlin inline function into {} brackets?

Kotlin inline function into {} brackets?

I have a function like:

private fun Logging.ILoggingEvent.populateLoggingEvent(eventId: String, data: MutableMap<String, String?> = mutableMapOf()) =
    LoggingEventData(
        eventId = eventId,
        eventName = "LoggingEvent_$eventId",
        data = data
    )

while:

data class LoggingEventData(
    val eventId: String,
    val eventName: String,
    val data: MutableMap<String, String?>,
)

and

interface ILoggingEvent {

    val timeStamp: Instant

    fun updateTimeStamp(now: Instant) {
        timeStamp = now
    }
}

Why can I not convert upper populateLoggingEvent() function into brackets:

private fun Logging.ILoggingEvent.populateLoggingEvent(eventId: String, data: MutableMap<String, String?> = mutableMapOf()) {
    LoggingEventData(
        eventId = eventId,
        eventName = "LoggingEvent_$eventId",
        data = data
    )
}

Solution

When a function declaration is of the form fun f() = expression, it means that f is a function that returns expression.

To rewrite this using a block as the function body, you need to

write the return type explicitly
rewrite the expression into a return statement.

This means adding : LoggingEventData before the { ... }, and adding return before LoggingEventData(...).

private fun Logging.ILoggingEvent.populateLoggingEvent(
    eventId: String, data: MutableMap<String, String?> = mutableMapOf()
): LoggingEventData {
    return LoggingEventData(
        eventId = eventId, 
        eventName = "LoggingEvent_$eventId", 
        data = data
    )
}

In fact, converting between expression-bodied and block-bodied functions is available as a context action in IntelliJ. Right click the expression and select "Show Context Actions" (or Cmd+Enter on macOS), and select "convert to block body".

In your attempt, the function does not have a return type, so it becomes a Unit-returning function. The expression LoggingEventData(...) in the body is simply creating a LoggingEventData, and then discards it.