Scala; Can you define many parameters as one

Question

Can you define a set of variables for later use?

Here are some pseudo code highlighting my intent:

def coordinates = x1, y1, x2, y2

log("Drawing from (%4.1f, %4.1f) to (%4.1f, %4.1f)".format(coordinates))
canvas.drawLine(coordinates, linePaint)

Here is a working example that contains duplicated code.

log("Drawing from (%4.1f, %4.1f) to (%4.1f, %4.1f)".format(x1, y1, x2, y2))
canvas.drawLine(x1, y1, x2, y2, linePaint)

Not sure what your intent is, maybe varags can help: http://stackoverflow.com/questions/4176440/syntax-sugar — Malte Schwerhoff, Jul 15 '12 at 20:27
Varags wont work. Varags must be the last parameter and thus isn't general. — , Jul 15 '12 at 20:34
Did you have a look at Scala macros? http://scalamacros.org/ They are an experimental feature, though. — Malte Schwerhoff, Jul 15 '12 at 20:46
Actually no as I'm on the 2.9.2 branch. But I will look into that on a later occasion. — , Jul 15 '12 at 20:53
Using macros for a problem like this seems a little excessive (and also probably a good way to make everyone who ever reads your code hate you). — Travis Brown, Jul 15 '12 at 21:23
How would macros help in this case? You’d then have to enhance the Android API with them. — Debilski, Jul 15 '12 at 21:29

score 5 · Answer 1 · answered Jul 15 '12 at 21:15

Yes, you can, although the syntax is arguably horribly clunky, and there are some limitations that may seem a little arbitrary at first. The trick is to convert the method to a function (called "eta expansion"), and then to use that function's tupled method to get something you can apply to a tuple.

Suppose you have a class like this:

class Foo {
  def f(a: String, b: String) = "%s, %s".format(b, a)
  def g(x: Int, y: Int, z: Int) = x + y * z
}

And an instance:

val foo = new Foo

And some data you'd like to use Foo's methods on:

val names = ("John", "Doe")
val nums = (42, 3, 37)

You can't just write foo.f(names) or foo.g(nums), because the types don't line up—argument lists and tuples are different things in Scala. But you can write the following:

scala> (foo.f _).tupled(names)
res0: String = Doe, John

scala> (foo.g _).tupled(nums)
res1: Int = 153

Sticking the underscore after the method turns it into a function (this is in my opinion the most confusing little quirk of Scala's syntax), and tupled converts it from a function with two (or three) arguments to a function with a single tuple argument.

You could clean the code up a little by defining the following helper functions, for example:

scala> val myF = (foo.f _).tupled
myF: ((String, String)) => String = <function1>

scala> val myG = (foo.g _).tupled
myG: ((Int, Int, Int)) => Int = <function1>

scala> myF(names)
res2: String = Doe, John

scala> myG(nums)
res3: Int = 153

I'm not sure that's much better, though.

Lastly, you can't (conveniently) use this approach on a varargs method—you can't for example write the following:

val coordsTupleToString = ("(%4.1f, %4.1f) to (%4.1f, %4.1f)".format _).tupled

Or even just:

val coordsToString = "(%4.1f, %4.1f) to (%4.1f, %4.1f)".format _

Which is yet another reason to avoid varargs in Scala.

Thanks very resourceful post. Can this approach be used while having a method that use the first 4 parameters from a tuple and then later by normal use? Like m(p1,p2,p3,p4,p5) written as m(Tuple4, p5) — , Jul 15 '12 at 21:31
You can use this on varargs just fine as long as they are happy with the `Any` type: `printf("%d %d %d %s", (1,2,3,"fish").productIterator.toSeq: _*)` — Rex Kerr, Jul 15 '12 at 21:35

score 2 · Answer 2 · answered Jul 15 '12 at 20:24

2

Looks like you need a tuple:

val coordinates = (x1, y1, x2, y2)

or maybe a full-blown object?

answered Jul 15 '12 at 20:24

Tomasz Nurkiewicz

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I don't think a `tuple` is what I'm after? Or can you auto expand them? Otherwise I have to do method(coordinates ._1, coordinates ._2, coordinates._3, coordinates._4) which isn't what I'm after – Jul 15 '12 at 20:33
1

@Farmor: if you have control over method you want to call this way, you can easily extract tuple with: `val (x1, y1, x2, y2) = coordinates` – Tomasz Nurkiewicz Jul 15 '12 at 20:37
No I don't have that as it's Androids API which exposes `drawLine` on a `Canvas`. I was primarily interested in if Scala was able to define a set of parameters and then use them in a conceptual find and replace way. – Jul 15 '12 at 20:40

Debilski · Answer 3 · 2012-07-15T21:42:55.593

Now, this may be obvious, but if it annoys you in only a few cases, you can always enhance:

implicit def enhancedCanvas(canvas: Canvas) = new {
  // using bad and slow syntax. please change this in Scala 2.10.
  def drawLineC(coordinates: (Float, Float, Float, Float), paint: Paint) = {
    val (x1, y1, x2, y2) = coordinates
    canvas.drawLine(x1, y1, x2, y2, paint)
  }
}

Another possibility, if you’re crazy enough. (Might be that an enhancement like this is already in Scalaz or Shapeless.)

implicit def enhTuple4[A,B,C,D](t: Tuple4[A,B,C,D]) = new {
  def |<[E] (f: (A, B, C, D) => E) = f(t._1, t._2, t._3, t._4)
}

// to be used as
val coordinates = (x1, y1, x2, y2)
coordinates |< (canvas.drawLine(_, _, _, _, linePaint))

Scala; Can you define many parameters as one

3 Answers3