From: http://www.jetbrains.com/mps/docs/tutorial.html

swing_calculator_dsl.rb

#! /usr/bin/env jruby
include Java

import java.lang.System
import javax.swing.JFrame
import javax.swing.JLabel
import javax.swing.JTextField
import javax.swing.event.DocumentListener
import java.awt.GridLayout

class Calculator < JFrame
  def initialize(name='Calculator', &block)
    super(name)
    self.layout = GridLayout.new(0, 2)
    @fields = {}
    
    configue(&block)
    
    pack
    show
  end
  
  def configue(&block)
    data = CalculatorDSL.new(&block)
    
    data.inputs.each do |input|
      add JLabel.new(input.to_s)
      add @fields[input] = JTextField.new
    end
    
    data.outputs.each do |output, spec|
      add JLabel.new(output.to_s)
      add field = JTextField.new
      field.enabled = false
      @fields[output] = field # can cause loops
      
      calculation = spec.pop
      compute = lambda{
        values = spec.map{|key| @fields[key].get_text.to_i }
        result = calculation.call(*values)
        field.set_text(result.to_s)
      }
      
      spec.each{|key| @fields[key].get_document.add_document_listener(FieldListener.new(compute))}
    end
    
  end
end

class CalculatorDSL
  attr_reader :inputs, :outputs
  def initialize(&block)
    @inputs = []
    @outputs = {}
    instance_eval &block
  end
  
  def input *names
    names.each{|name| @inputs << name }
  end
  
  def output specs
    specs.each{|name, spec| @outputs[name] = spec}
  end
end

class FieldListener
  include DocumentListener
  
  def initialize(block)
    @block = block
  end
  
  def insertUpdate(e); update; end  
  def removeUpdate(e); update; end
  def changedUpdate(e); update; end
  def update; @block.call; end
end

# This is the equivelant DSL
Calculator.new do
  input   :width
  input   :height
  output  :area =>  [:width, :height, lambda{|w,h| w * h}]
  output  :price => [:area, lambda{|area| "$#{area * 25}" }]
end