Scala functions

Developers coming into Kotlin from Scala may sometimes define their function this way:

fun hello() = {
    "hello"
}

Calling this function won't print what you expect:

println("Say ${hello()}")

It prints the following:

 Say () -> kotlin.String

What we're missing is the second set of parentheses:

println("Say ${hello()()}")

It prints the following:

Say hello

That's because the single-expression definition could be translated into:

fun hello(): () -> String {
    return {
        "hello"
    }
}

It could be further translated into:

fun helloExpandedMore(): () -> String {
    return fun(): String {
        return "hello"
    }
}

Now you can see where that function came from, at least.