I have a DSL function whose lambda returns `Any?` ...

# dsl

[jw]

I have a DSL function whose lambda returns Any? allowing any object to be returned. This lambda's receiver is also a DSL type meaning there are functions you can call inside of it. The problem is that these functions return Unit thus completing the contract of the enclosing lambda and avoiding the need to actually return an object. Is there any way to avoid this?

@MyDsl interface Foo {
  fun bar(lambda: Bar.() -> Any?)
}
@MyDsl interface Bar {
  fun someHelper(size: Int)
  fun otherHelper(flag: Boolean)
}

bar {
  someHelper(2)
  otherHelper(true)
  // should be required to return an object here
}

I tried to get clever with an overload of bar whose lambda returned Unit and was marked as an error, but you can't @JvmName an interface function. converting it to a class results in the Unit-returned lambda always being preferred. There's a Kotlin-internal annotation to lower its overload precedence, but I don't have access to that.

[spand]

I got a bit further by taking both Any? and Unit methods out of the interface letting you JvmName them it seems return type inference isnt quite up to it.

@MyDsl interface Foo {
    fun internalMethod()
}
@JvmName("asdasd")
@Deprecated("You must return something", level = DeprecationLevel.ERROR)
fun Foo.bar(lambda: Bar.() -> Unit) = Unit

@JvmName("asd")
fun Foo.bar(lambda: Bar.() -> Any?){
    internalMethod()
}


@MyDsl interface Bar {
    fun someHelper(size: Int)
    fun otherHelper(flag: Boolean)
}

fun Foo.f() {
    bar {
        someHelper(2)
        otherHelper(true)
        // should be required to return an object here
        "2"
    }
}

[spand]

Best I got was something like this:

@MyDsl interface Foo {
    fun internalMethod()
}

@JvmName("asd")
fun <R> Foo.bar(lambda: Bar.() -> R){
    internalMethod()
}


@MyDsl interface Bar {
    fun someHelper(size: Int)
    fun otherHelper(flag: Boolean)
}

fun Foo.f() {
    bar<String> {
        someHelper(2)
        otherHelper(true)
        // should be required to return an object here
//        "2"
    }
}