Selecting function reference before application: problem with type inference

Question

I want to choose function reference before applying it to arguments, but Kotlin cannot infer its type.

Suppose I have a class

class Circle(val radius: Float)

and a function

fun sortByRadius(circles: MutableList<Circle>, ascending: Boolean) {
    if (ascending)
        circles.sortBy { it.radius }
    else
        circles.sortByDescending { it.radius }
}

I want too rewrite this function body to something like this:

circles.(
    if (ascending) MutableList<Circle>::sortBy
    else MutableList<Circle>::sortByDescending
) { it.radius }

but it does not work. Also I found out that

(MutableList<Circle>::sortBy)(circles) { it.radius }

almost works, Kotlin just can not infer Float type of radius; so I wonder how to specify it. Then we could write

(if (ascending) MutableList<Circle>::sortBy
    else MutableList<Circle>::sortByDescending)(circles) { it.radius }

Answer 1

Nice question, and it doesn't seem to be possible (using function references). I've tried to specify expected type explicitly:

val f: MutableList<Circle>.((Circle) -> Float) -> Unit = 
    if (ascending) MutableList<Circle>::sortBy else MutableList<Circle>::sortByDescending

but the error message says

Type inference failed. Please try to specify type arguments explicitly.

and the grammar doesn't allow it:

callableReference
  : (receiverType? '::' (simpleIdentifier | 'class'))
  ;

i.e. :: can only be followed by an identifier or class. Which is a strange hole because Java grammar does allow type parameters before method name.

Bound references don't help either:

(circles::sortBy) { it.radius } // doesn't compile

Not even explicit type works, which quite surprised me:

val f: ((Circle) -> Float) -> Unit = circles::sortBy 

but this does:

val f: ((Circle) -> Float) -> Unit = { circles.sortBy(it) }

So ultimately you can write (parentheses around lambdas required)

val f: ((Circle) -> Float) -> Unit = 
  if (ascending) ({ circles.sortBy(it) }) else ({ circles.sortByDescending(it) })
f { it.radius }

but probably not want to!