Can a partially applied function be extracted or interrogated for the applied parameter value - scala

Question

Is there a way to extract or interrogate a partially applied function to get the applied value.
For example, can the value 3 be extracted from reduceBy3 in the code below.
def subtract(x:Int, y:Int) = x-y
val reduceBy3 = subtract(3,_:Int)
I have experimented with creating an extractor has shown in the example below however the unapply method must accept an (Int=>Int) function that requires interrogation.
class ReduceBy(y: Int) {
val amt = y
def subtract(y: Int, x: Int) = x - y
}
object ReduceBy extends Function1[Int, Int => Int] {
def apply(y: Int) = {
val r = new ReduceBy(y)
r.subtract(y, _: Int)
}
def unapply(reduceBy: ReduceBy): Option[Int] = Some(reduceBy.amt)
}
object ExtractPartialApplied extends App {
val r3 = ReduceBy(3)
val extract = r3 match {
case ReduceBy(x) => ("reduceBy", x)
case x: ReduceBy => ("reduceBy", x.amt)
case _ => ("No Match", 0)
}
println(extract)
val z = r3(5)
println(z)
}

Answer 1

You can have your subtract method receive the first parameter, and then return a function-like object which will then take the second parameter, similarly to a multiple-argument-list function, but which you can then extend however you wish.
This doesn't look very elegant though, and needs a bit of manual boilerplate.
class ReduceBy(val amt: Int) {
def subtract(x: Int) = {
val xx = x // avoid shadowing
new Function[Int, Int] {
def x = xx
def apply(y: Int) = x - y
}
}
}

Answer 2

A solution adapting the answer by danielkza is to have the companion object do the extraction and return a ReduceBy function that holds onto the the initial value.
object ReduceBy {
def apply(y: Int) = new ReduceBy(y)
def unapply(reduceBy: ReduceBy): Option[Int] = Some(reduceBy.amt)
}
class ReduceBy(val amt: Int) extends Function[Int, Int] {
def apply(y: Int) = y - amt
}
object ExtractPartialApplied extends App {
val reduceBy3 = ReduceBy(3)
val extract = reduceBy3 match {
case ReduceBy(x) => ("ReduceBy(x)", x)
case x: ReduceBy => ("ReduceBy", x.amt)
case _ => ("No Match", 0)
}
println(extract)
println(reduceBy3(5))
}