Kotlin - Restrict extension method scope

Question

Is there a way to restrict extension methods in DSLs?

Say I have a class structure like this:

class Outer {
    fun middle(op: Middle.() -> Unit): Middle {
        val middle = Middle()
        middle.op()
        return middle
    }
}

class Middle {
    fun inner(op: Inner.() -> Unit): Inner {
        val inner = Inner()
        inner.op()
        return inner
    }
}

class Inner

fun outer(op: Outer.() -> Unit): Outer {
    val outer = Outer()
    outer.op()
    return outer
}

I can then create a call like so:

outer {
    middle {
        inner {
            middle { }  // Here is the problem
        }
    }
}

My problem is that the marked middle { } call is confusing, as it adds a Middle to the Outer when it looks like it is adding to the Inner.

Is there a way to not allow the middle { } call?

Answer 1

You can use a workaround with deprecated:

class Outer {
    fun middle(op: Middle.() -> Unit): Middle {...}

    @Deprecated("can not be used inside a Outer block", level = DeprecationLevel.ERROR)
    fun outer(op: Outer.() -> Unit): Outer = TODO()
}

class Middle {
    fun inner(op: Inner.() -> Unit): Inner {...}

    @Deprecated("can not be used inside a Middle block", level = DeprecationLevel.ERROR)
    fun middle(op: Middle.() -> Unit): Middle = TODO()
}

class Inner {
    @Deprecated("can not be used inside a Inner block", level = DeprecationLevel.ERROR)
    fun inner(op: Inner.() -> Unit): Inner = TODO()
}

Now the compiler will give you an error, and IDE will not suggest a wrong function in the completion:

outer {
    middle {
        inner {
            middle { }  // error
        }
    }
}

But you really should not do it for big DSLs. It is better to wait for https://youtrack.jetbrains.com/issue/KT-11551 as @KirillRakhman suggested.

Edit: After I fixed my example, it became much smaller. With one dummy function for a class it is not that much of boilerplate after all.

Answer 2

The official way to restrict scope is DslMarker. It cannot help in some cases (when you need to annotate java sources, for example) - and here @Deprecated is used. But try DslMarker first.

@DslMarker
@Target(AnnotationTarget.CLASS, AnnotationTarget.TYPE)
annotation class Scope

@Scope
class Outer {
    fun middle(op: Middle.() -> Unit): Middle { /**/ }
}

@Scope
class Middle {
    fun inner(op: Inner.() -> Unit): Inner {/**/ }
}

class Inner

Thus the last middle call is not compilable anymore.