Explicitly returning from lambda expression in Kotlin

Question

Using Spark, I end up with a bit of code like this:

post("/auth/login", { req, res ->
    val body = parseBody(req.body())
    val assertion = body["assertion"]
    if (assertion == null) {
        halt(400)
        return null
    }
    // ...snip...lots more code
})

Which works great, except...it doesn't compile -- I get 'return' is not allowed here.

I could put the remainder of the lambda in an else block, but I'd rather not in the interest of minimizing indentation.

So how do I "short circuit" the lambda to return a null?

Answer 1

In my understanding, Lambda cant use return, return is for the function by default.

So you need to use a label to tell the return clause where it will return to. I use this:

f@ {
 ...
 return@f null
}