Question

Can I retrieve a Method via reflection, somehow combine it with a target object, and return it as something that looks like a function in Scala (i.e. you can call it using parenthesis)? The argument list is variable. It doesn’t have to be a “first-class” function (I’ve updated the question), just a syntactic-looking function call, e.g. f(args).

My attempt so far looks something like this (which technically is pseudo-code, just to avoid cluttering up the post with additional definitions):

class method_ref(o: AnyRef, m: java.lang.reflect.Method) {
    def apply(args: Any*): some_return_type = {
        var oa: Array[Object] = args.toArray.map { _.asInstanceOf[Object] }
        println("calling: " + m.toString + " with: " + oa.length)

        m.invoke(o, oa: _*) match  {
            case x: some_return_type => x;
            case u => throw new Exception("unknown result" + u);
        }
    }
}

With the above I was able to get past the compiler errors, but now I have a run-time exception:

Caused by: java.lang.IllegalArgumentException: argument type mismatch

The example usage is something like:

  var f = ... some expression returning method_ref ...;
  ...
  var y = f(x) // looks like a function, doesn't it?

UPDATE

Changing the args:Any* to args:AnyRef* actually fixed my run-time problem, so the above approach (with the fix) works fine for what I was trying to accomplish. I think I ran into a more general issue with varargs here.

Answer 1

If you’re not looking for a generic invoke that takes the method name–but rather, you want to capture a particular method on a particular object–and you don’t want to get too deeply into manifests and such, I think the following is a decent solution:

class MethodFunc[T <: AnyRef](o: Object, m: reflect.Method, tc: Class[T]) {
  def apply(oa: Any*): T = {
    val result = m.invoke(o, oa.map(_.asInstanceOf[AnyRef]): _*)
    if (result.getClass == tc) result.asInstanceOf[T]
    else throw new IllegalArgumentException("Unexpected result " + result)
  }
}

Let’s see it in action:

val s = "Hi there, friend"
val m = s.getClass.getMethods.find(m => {
  m.getName == "substring" && m.getParameterTypes.length == 2
}).get
val mf = new MethodFunc(s,m,classOf[String])

scala> mf(3,8)
res10: String = there

The tricky part is getting the correct type for the return value. Here it’s left up to you to supply it. For example,if you supply classOf[CharSequence] it will fail because it’s not the right class. (Manifests are better for this, but you did ask for simple…though I think “simple to use” is generally better than “simple to code the functionality”.)