#from http://weblog.raganwald.com/2007/03/approach-to-composing-domain-specific.html


		
	#!/usr/bin/env ruby
	#
	#  Created by Reginald Braithwaite on 2007-03-11.
	#  Copyright (c) 2007. All rights reserved.
	#
	#            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
	#                    Version 2, December 2004
	# 
	# Copyright (C) 2004 Sam Hocevar
	#  22 rue de Plaisance, 75014 Paris, France
	# Everyone is permitted to copy and distribute verbatim or modified
	# copies of this license document, and changing it is allowed as long
	# as the name is changed.
	# 
	#            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
	#   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
	# 
	#  0. You just DO WHAT THE FUCK YOU WANT TO.
	
	# A Domain Specific Language is used to introduce a new scope with an embedded set of methods.
	#
	# The idea is to avoid polluting the global namespace. Instead of adding methods to Kernel, we
	# add methods to a new DomainSpecificLanguage, and then we can evaluate code with the new language
	# using the #eval class method or using the #with method added to Kernel.
	#
	# For a similar approach, see http://www.infoq.com/articles/eval-options-in-ruby
	
	class DomainSpecificLanguage

	  # See http://whytheluckystiff.net/articles/seeingMetaclassesClearly.html
	
	  def metaclass; class << self; self; end; end
	  def meta_eval &blk; metaclass.instance_eval &blk; end

	  # Adds methods to a metaclass
	  def meta_def name, &blk
	    meta_eval { define_method name, &blk }
	  end

	  # See http://onestepback.org/index.cgi/Tech/Ruby/RubyBindings.rdoc
	  
	  class ReadOnlyReference
	    def initialize(var_name, vars)
	      @getter = eval "lambda { #{var_name} }", vars
	    end
	    def value
	      @getter.call
	    end
	  end

	  attr_reader :outer_binding, :outer_self

	  # instances of a DomainSpecificLanguage are created each time
	  # a block is evaluated with that language. The instance is
	  # intialized with the block's binding.
	  
	  def initialize(given_binding)
	    @outer_binding = given_binding
	    @outer_self = ReadOnlyReference.new(:self, given_binding)
	  end

	  # some jiggery-pokery to access methods defined in the block's
	  # scope, because when the block is evaluated with the DomainSpecificLanguage,
	  # we use #instance_eval to set <tt>self</tt> to the DomainSpecificLanguage
	  # instance.
	  def method_missing(symbol, *args, &block)
	    if args.empty?
	      r = ReadOnlyReference.new(symbol, outer_binding)
	      meta_def(symbol) { r.value }
	      r.value
	    else
	      outer_self.value.send symbol, *args, &block
	    end
	  end

	  class << self

	    # Evaluates a block in the context of a new DomainSpecificlanguage
	    # instance.
	    def eval &block
	      new(block.binding).instance_eval(&block)
	    end

	  end

	end

	# We open Kernel and add just one method for introducing DomainSpecificLanguages
	
	module Kernel

	  # Evaluate a block with a DomainSpecificLanguage
	  
	  def with dsl_class, &block
	    dsl_class.eval(&block)
	  end

	end

	# Let is a DomainSpecificLanguage that actually creates DomainSpecificLanguages.
	#
	# Let works a lot like <tt>let</tt> in Scheme. Your provide a hash of names and value
	# expressions. The value expressions are evaluated in the outer scope, and then we have
	# a little domain specific language wher ethe inner scope has the names all bound to the
	# values. E.g.
	# <tt>
	# with Let do
	#     let :x => 100, :y => 50 do
	#         print "#{x + y} should equal fifty"
	#     end
	# end
	# </tt>
	#
	# Within the Let DomainSpecificLanguage, you can declare multiple <tt>let</tt> statements and nest
	# them as you please.
	#
	# One important limitation: you cannot bind a value to a name that is already a local variable.
	
	class Let < DomainSpecificLanguage

	  class Scope < DomainSpecificLanguage

	    # initializes a Scope. In addition to the outer binding, we also pass in the
	    # hash of names and values. Note the check to ensure we are not trying to
	    # override a lcoal variable.
	    
	    def initialize given_binding, let_clauses = {}
	      let_clauses.each do |symbol, value| 
	        var_name = symbol.to_s
	        raise ArgumentError.new("Cannot override local #{var_name}") if eval("local_variables", given_binding).detect { |local| local == var_name  }
	        meta_eval { attr_accessor(var_name) }
	        send "#{var_name}=", value
	      end
	      super(given_binding)
	    end

	  end

	  # Define a new Scope: you're really defining a new DomainSpecificLanguage
	  
	  def let let_clauses = {}, &block
	    Scope.new(block.binding, let_clauses).instance_eval(&block)
	  end

	  class << self

	    # If you just want a one-off
	    # def eval let_clauses = {}, &block
	    #   Scope.new(block.binding, let_clauses).instance_eval(&block)
	    # end
	    
	  end

	end

	# A DomainSpecificDelegator is a DSL that delegates methods to a class or object.
	# The main use is to separate the mechanics of scoping from the methods of a utility
	# class.
	
	class DomainSpecificDelegator < DomainSpecificLanguage

	  class << self

	    # insert one or more #delegate_to calls in the class definition, giving a receiver
	    # and a hash. Each hash pair is of the form <tt>verb => method</tt> where verb is the
	    # name you will use in the DSL and method is the method in the receiver that will handle
	    # it.
	    def delegate_to receiver, method_hash
	      @@delegations ||= {}
	      method_hash.each { |verb, method_name| @@delegations[verb.to_s] = [receiver, method_name.to_s] }
	    end

	  end

	  def method_missing symbol, *args, &block
	    receiver, method_name = *@@delegations[symbol.to_s]
	    if receiver
	      receiver.send method_name, *args, &block
	    else
	      super(symbol, *args, &block)
	    end
	  end

	end
	
	

	
	#!/usr/bin/env ruby
	#
	#  Created by Reginald Braithwaite on 2007-03-11.
	#  Copyright (c) 2007. All rights reserved.
	#
	#            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
	#                    Version 2, December 2004
	# 
	# Copyright (C) 2004 Sam Hocevar
	#  22 rue de Plaisance, 75014 Paris, France
	# Everyone is permitted to copy and distribute verbatim or modified
	# copies of this license document, and changing it is allowed as long
	# as the name is changed.
	# 
	#            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
	#   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
	# 
	#  0. You just DO WHAT THE FUCK YOU WANT TO.
	
	require "test/unit"
	require "dsl"

	class DefinesVariable < DomainSpecificLanguage
	  def y
	    5
	  end
	end

	class DefinesMethodWithBlock  < DomainSpecificLanguage
	  def say something
	    something
	  end
	end

	class TestDomainSpecificLanguage < Test::Unit::TestCase

	  def test_simple_read
	    x = 100
	    assert_equal(500, DomainSpecificLanguage.eval { x * 5 })
	  end

	  def test_define_reader
	    x = 100
	    assert_equal(500, DefinesVariable.eval { x * y })
	  end

	  def test_define_method
	    assert_nothing_raised(Exception) { DefinesMethodWithBlock.eval { say 'hello' } }
	  end

	  def test_uses_lambda
	    double = lambda { |x| x * 2 }
	    assert_equal(8, DomainSpecificLanguage.eval { double.call(4) })
	  end

	  def square x
	    x * x
	  end

	  def test_uses_method
	    assert_equal(16, DomainSpecificLanguage.eval { square(4) })
	  end

	end

	class DomainSpecificDelegatorTestHelper < DomainSpecificDelegator

	  delegate_to "hello", :say => :to_s

	end

	class TestDomainSpecificDelegator < Test::Unit::TestCase

	  def test_say
	    with DomainSpecificDelegatorTestHelper do
	      assert_equal('hello', say)
	    end
	  end

	  def world
	    'world'
	  end

	  def test_outer_scope_method
	    with DomainSpecificDelegatorTestHelper do
	      assert_equal('hello world', "#{say} #{world}")
	    end
	  end

	end

	class TestLet < Test::Unit::TestCase

	  def test_cannot_try_to_override_locals
	    with Let do
	      local_var = :snafu
	      assert_raise(ArgumentError) do
	        let :local_var => :fubar do 
	          fail();
	        end
	      end
	    end
	  end

	  def test_scoping
	    with Let do
	      let :x => 1, :y => 2 do
	        assert_equal(1, x) 
	        assert_equal(2, y)
	      end
	    end
	  end

	  def test_access_to_outer_locals
	    with Let do
	      x = 1
	      let :y => 2 do
	        assert_equal(3, x + y)
	      end
	    end
	  end

	  def z
	    -1
	  end

	  def test_access_to_outer_methods
	    with Let do
	      let :x => 1 do
	        assert_equal(0, x + z)
	      end
	    end
	  end

	  def test_nesting
	    with Let do
	      let :x => 0, :y => 1 do
	        assert_equal(1, x + y)
	        let :x => 2 do
	          assert_equal(3, x + y) 
	        end
	        assert_equal(0, x)
	      end 
	    end
	  end

	  def test_assignment
	    with Let do
	      let :x => 0, :y => 1 do
	        assert_equal(1, x + y)
	        let :x => 2 do
	          assert_equal(3, x + y) 
	          x = 5
	          assert_equal(5, x)
	          assert_equal(2, x()) # assignment creates a local that shadows the method.
	          self.x = 7
	          assert_equal(5, x)
	          assert_equal(7, x()) # you can hack around the read only nature, but it's not pretty
	        end
	        assert_equal(0, x)
	      end
	    end
	  end

	  def test_let_and_block_parameters
	    with Let do
	      let :x => nil do
	        (1..10).each { |x| assert !x.nil? }
	        assert_nil(x)
	      end
	    end
	  end

	  attr_accessor :w

	  def test_outer_assigment
	    w = 5
	    with Let do
	      let :x => 5 do
	        assert_equal(25, w * x)
	        w = 7
	        assert_equal(35, w * x)
	      end
	    end
	    assert_equal(7, w)
	  end

	end