traits

# A trait: a set of methods a type promises to have.

trait Shape
  # `abstract def` is a requirement. Every implementer must define it.
  abstract def area : Float64
end

struct Circle
  def initialize(@radius : Float64)
  end
end

struct Square
  def initialize(@side : Float64)
  end
end

# `impl Trait for Type` is the one way to give a type a trait. The impl
# lives next to the trait or next to the type, never anywhere else.
impl Shape for Circle
  def area : Float64
    3.14159 * @radius * @radius
  end
end

impl Shape for Square
  def area : Float64
    @side * @side
  end
end

puts Circle.new(1.0).area.round(2)
puts Square.new(2.0).area

# Leave a requirement out and the compiler refuses the impl, naming the
# missing method. Try removing `area` from one of the impls above.