Checking the arity of a method object - scala

Question

Given any method, eg
def add(x: Int, y: Int) = {
x + y
}
Can I use any inspection/reflection library to obtain the arity of add?
Something like arity(add), since the function object does not seems to have a property to provide that information.

Answer 1

One type-safe solution is to overload your Node's constructor for each Function* trait that corresponds to the arity of a function that you will pass as a parameter:
scala> class Node {
| def this(f: Function0[Int]) = { this(); println(0) }
| def this(f: Function1[Int, Int]) = { this(); println(1) }
| def this(f: Function2[Int, Int, Int]) = { this(); println(2) }
| }
defined class Node
scala> new Node(add _)
2
res7: Node = Node#427128a6
If your situation requires a reflection-based approach somehow, you can count the number of parameters of the apply method that all the Function* traits share, as follows:
scala> def arity(f: AnyRef): Option[Int] = {
| val apply = f.getClass.getMethods.find(_.getName == "apply")
| apply.map(_.getParameterCount)
| }
arity: (f: AnyRef)Option[Int]
scala> arity(add _)
res0: Option[Int] = Some(2)
Or you might want to consider using typed patterns:
def arity(f: AnyRef): Int = f match {
case _: Function0[_] => 0
case _: Function1[_, _] => 1
case _: Function2[_, _, _] => 2
...
}