patrones.adoc

Patrones de diseño

Visitor

Example 1. Cómputo sobre un arbol sintáctico

Imprimir el resultado de un cierto cómputo (por ejemplo, multiplicar por dos) sobre todos los valores de un árbol sintáctico AST (Abstract Syntax Tree)

Doblar nodos - versión 1

class IntegerNode
  def initialize(value)
    @value = value
  end

  def double
    @value * 2
  end
end

class Ast
  def initialize
    @nodes = []
    @nodes << IntegerNode.new(2)
    @nodes << IntegerNode.new(3)
  end

  def print_double
    @nodes.each do |node|
      puts node.double
    end
  end
end

ast = Ast.new
ast.print_double # => 4 6

Above solution works. Now let’s try to print triple the value. In order to do that we need to change class IntegerNode. And IntegerNode has knowledge of how to print triple value. Tomorrow if we have another node called FloatNode then that node will have knowledge about how to double and triple the value.

Nodes are merely storing information. And the representation of data should be separate from the data itself. So IntegerNode and FloatNode should not know about how to double and triple.

To take the data representation code out of nodes we can make use of visitor pattern . Visitor pattern uses double dispatch .

Before we look at “double dispatch” let’s first look at “single dispatch”.