It would be great for DSL if I can state for a fun...

# language-proposals

[Marc Knaup]

It would be great for DSL if I can state for a function that at least one of the optional parameters must be specified, i.e. the function is otherwise either useless or an invocation invalid.

interface SomeType

@RequiresAtLeastOneOptionalArgument
fun SomeType.with(key: String? = null, ref: Ref<*>? = null, block: () -> Unit): Unit = TODO()

fun foo(some: SomeType) {
	some.with {
		// useless with
	}
	some.with(key = "key") {
		// good
	}
}

Another example:

@RequiresAtLeastOneOptionalArgument
fun makeBorder(width: BorderWidth? = null, style: BorderStyle? = null, color: Color? = null): Border =
	if (width != null || style != null || color != null) Border(width, style, color)
	else error("At least one property must be specified.")

makeBorder() is then statically known to be invalid.
It won’t catch makeBorder(null) but that’s another issue.

[ephemient]

dunno if there's a better way, but with O(n) overloads you can force any one parameter to be given a non-null value

private fun makeBorderImpl(width: BorderWidth?, style: BorderStyle?, color: Color?): Border = ...

fun makeBorder(width: BorderWidth, style: BorderStyle? = null, color: Color? = null) =
    makeBorderImpl(width = width, style = style, color = color)
fun makeBorder(style: BorderStyle, color: Color? = null) =
    makeBorderImpl(width = null, style = style, color = color)
fun makeBorder(color: Color) =
    makeBorderImpl(width = null, style = null, color = color)

[Marc Knaup]

Yeah that’s an option. But I have plenty of functions, so it’s not just n overloads but n * m (times amount of functions). That’s a looot.

(You also wouldn’t be able to call that with three nullable variables in your example)

[louiscad]

You can make a no argument overload that is deprecated with error level, with a clear message.