Question

Is there a way in scala to get the arguments back from a already partially applied function?

Does this even make sense, should be done, or fits into any use case?

example:


def doStuff(lower:Int,upper:Int,b:String)=
for(turn <- lower to upper) println(turn +": "+b)


Imagine that at one point I know the ‘lower’ argument and I get a function of applying it to ‘doStuff’


val lowerDoStuff = doStuff(3,_:Int,_:String)


Is there a way for me to get that 3 back ?
(for the sake of example, imagine that I am inside a function which only received ‘lowerDoStuff’ and now needs to know the first argument)

Idiomatic scala is prefered to introspection/reflection (if possible).

Answer 1

Idiomatic Scala: no, you can’t. You have specifically said that the first argument is no longer relevant. If the compiler can make it disappear entirely, that’s best: you say you have a function that depends on an int and a string, and you haven’t made any promises about what generated it. If you really need that value, but you also really need to pass a 2-argument function, you can do it by hand:

class Function2From3[A,B,C,Z](f: (A,B,C) => Z, val _1: A) extends Function2[B,C,Z] {
  def apply(b: B, c: C) = f(_1, b, c)
}
val lowerDoStuff = new Function2From3(doStuff _, 3)

Now when you get the function later on, you can pattern match to see if it’s a Function2From3, and then read the value:

val f: Function2[Int,String,Unit] = lowerDoStuff
f match {
  case g: Function2From3[_,_,_,_] => println("I know there's a "+g._1+" in there!")
  case _ => println("It's all Greek to me.")
}

(if it’s important to you that it be an integer, you can remove A as a generic parameter and make _1 be an integer–and maybe just call it lower while you’re at it).

Reflection: no, you can’t (not in general). The compiler’s smarter than that. The generated bytecode (if we wrap your code in class FuncApp) is:

public final void apply(int, java.lang.String);
  Signature: (ILjava/lang/String;)V
  Code:
   0:   aload_0
   1:   getfield    #18; //Field $outer:LFuncApp;
   4:   iconst_3
   5:   iload_1
   6:   aload_2
   7:   invokevirtual   #24; //Method FuncApp.doStuff:(IILjava/lang/String;)V
   10:  return

Notice the iconst_3? That’s where your 3 went–it disappeared into the bytecode. There’s not even a hidden private field containing the value any more.