Question

I’m interfacing scala to some native code (through java+jna) and am about to rewrite the api using tuples and multiple return parameters. But I’m wondering if there’s a better way (with implicits or something).

For example, I may have a method signature like

MyLib.__stdcall aMethod(String inParam, String[] out1, int[] out2);

Where the arrays are there so I can get the values in java:

String[] out1 = new String[1];
Int[] out2 = new Int[1];

lib.aMethod("in", out1, out2);

String theRealOut1Value = out1[0];
Int theRealOut2Value = out2[0];

and Voila! How simple!

I’m pretty sure the only reasonable thing to do is make a wrapper with a (scala) method sig like:

aMethodWrapper(in : String) : (String, Int)

I’m not hopeful, but I’m seeing magic at every turn in scala, so I was wondering 🙂

Answer 1

You can create the following helper function:

def call[A](f: Array[A] => Unit)(implicit m: Manifest[A]): A = {
  val outA = new Array[A](1)
  f(outA)
  outA(0)
}

…and for two arguments:

def call[A, B](f: (Array[A], Array[B]) => Unit)
             (implicit m1: Manifest[A], m2: Manifest[B]): (A,  B) = {
  val outA = new Array[A](1)
  val outB = new Array[B](1)
  f(outA, outB)
  (outA(0), outB(0))
}

After that you may call the library functions like this:

call[String](lib.aMethod("foo", _))

and

call[String, Int](lib.bMethod("bar", 1, _, _))