Scala: Naming parameters of function parameters

Question

I have an Image trait that represents a 2D ARGB image. Image has a map method which takes a mapping function and transforms the image using said function. The mapping function has 3 parameters: the X and Y coordinates and the color of the image at that coordinate. The color is represented as a 32-bit ARGB value packed into an Int.

trait Image {
    def map(f: (Int, Int, Int) => Int)
}

However, without a comment, it's impossible to tell which parameter of f is which.

In C#, I would create a delegate for this, which allows me to name the parameters of the mapping function:

delegate int MapImage(int x, int y, int color);

Is there anything of this sort in Scala? Is it being considered for addition to the language? Without it, I cannot write an interface that is readable without explicit documentation.

(Note: I know I should wrap the Int in a case class for the purposes of representing a color, but this is just an illustrative example.)

Possible duplicate of How can you make custom function types in Scala with named parameters? — the21st
– the21st, Commented May 19, 2016 at 12:46

lmm · Accepted Answer · 2015-06-25 10:55:19Z

1

You could declare a trait that f implements. With SAM support (enabled if you compile with -Xexperimental, and will be in the next version) this should be just as easy to use.

trait ImageMapper {
  def mapImage(x: Int, y: Int, color: Int): Int
}

trait Image {
  def map(f: ImageMapper) = ...
}

myImage.map{ (x, y, color) => ... } //the anonymous function
// is automatically "lifted" to be an implementation of the trait.

answered Jun 25, 2015 at 10:55

lmm

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1 Comment

the21st Over a year ago

I didn't know about SAM, it sounds like something I was looking for. Thanks!

bjfletcher · Accepted Answer · 2015-06-24 16:26:46Z

0

Perhaps through using types:

trait Image {
  type x = Int
  type y = Int
  type color = Int

  def map(f: (x, y, color) => Int)
}

I'd much rather using case class like so:

case class MapImage(x: Int, y: Int, color: Int)
trait Image {
  def map(f: MapImage => Int)
}

edited Jun 24, 2015 at 16:26

answered Jun 24, 2015 at 16:09

bjfletcher

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