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module demo.Shapes

# A struct is a named record. Its fields are read with a colon: p: x().
struct Point = { x, y }

# A union is one type with several forms. Each variant may carry fields, and
# every variant gets an is<Variant>() predicate for free.
union Shape = {
  Circle = { radius }
  Rect = { width, height }
}

# An augmentation adds functions to a type after the fact. `this` is the value
# the function is called on.
augment Point {
  function describe = |this| -> "(" + this: x() + ", " + this: y() + ")"
}

augment Shape {
  function area = |this| {
    return match {
      when this: isCircle() then 3.14159 * this: radius() * this: radius()
      when this: isRect() then this: width() * this: height()
      otherwise 0
    }
  }
}

function main = |args| {
  let origin = Point(0, 0)
  println("origin is " + origin: describe())

  let shapes = list[Shape_Circle(1.0), Shape_Rect(2.0, 3.0)]
  foreach shape in shapes {
    println(shape + " has area " + shape: area())
  }

  # A closure is a function value: |parameters| -> expression.
  let scaled = list[shape: area() * 2 foreach shape in shapes]
  println("doubled: " + scaled)
}