Visitor pattern from "Groovy in Action"

class Drawing {
    List shapes
    def accept(Closure yield) { shapes.each { it.accept(yield) } }
}

class Shape {
    def accept(Closure yield) { yield(this) }
}

class Square extends Shape {
    def width
    def area() { width ** 2 }
}

class Circle extends Shape {
    def radius
    def area() { Math.PI * radius ** 2 }
}

def picture = new Drawing(shapes:[new Square(width:1), new Circle(radius:1)])
def total = 0

picture.accept { total += it.area() }

println "The shapes in this drawing cover an area of $total"
println "The individual contributions are: "
picture.accept { println it.class.name + ":" + it.area() }