Methods

In a previous chapter we used this program

val box =
  Image.rectangle(40, 40).
    strokeWidth(5.0).
    strokeColor(Color.royalBlue.spin(30.degrees)).
    fillColor(Color.royalBlue)

box.beside(box).beside(box).beside(box).beside(box)

to create this image

Imagine we wanted to change the color of the boxes. Right now we would have to write out the expression again for each different choice of color.

val paleGoldenrod = {
  val box =
    Image.rectangle(40, 40).
      strokeWidth(5.0).
      strokeColor(Color.paleGoldenrod.spin(30.degrees)).
      fillColor(Color.paleGoldenrod)

  box.beside(box).beside(box).beside(box).beside(box)
}

val lightSteelBlue = {
  val box =
    Image.rectangle(40, 40).
      strokeWidth(5.0).
      strokeColor(Color.lightSteelBlue.spin(30.degrees)).
      fillColor(Color.lightSteelBlue)

  box.beside(box).beside(box).beside(box).beside(box)
}

val mistyRose = {
  val box =
    Image.rectangle(40, 40).
      strokeWidth(5.0).
      strokeColor(Color.mistyRose.spin(30.degrees)).
      fillColor(Color.mistyRose)

  box.beside(box).beside(box).beside(box).beside(box)
}

This is tedious. Each expression only differs in a minor way. It would be nice if we could capture the general pattern and allow the color to vary. We can do exactly this by declaring a method.

def boxes(color: Color): Image = {
  val box =
    Image.rectangle(40, 40).
      strokeWidth(5.0).
      strokeColor(color.spin(30.degrees)).
      fillColor(color)

  box.beside(box).beside(box).beside(box).beside(box)
}

// Create boxes with different colors
boxes(Color.paleGoldenrod)
boxes(Color.lightSteelBlue)
boxes(Color.mistyRose)

Try this yourself to see that you get the same result using the method as you did writing everything out by hand.

Now that we've seen an example of declaring a method, we need to explain the syntax of methods. Next, we'll look at how to write methods, the semantics of method calls, and how they work in terms of substitution.