Defines `reverse_evaluate`, which takes a block and evaluates its contents in reverse order.

reverse_evaluate.rb

# Defines a macro in Ruby which takes a block, and evaluates the
# block's statements in reverse order.
#
# Requires the 'live_ast' and 'ruby2ruby' gems. Install via:
#
#   gem install live_ast ruby2ruby
#
# Should work in 1.8 as well as 1.9.

require 'rubygems'
require 'live_ast'
require 'ruby2ruby'

def forward_evaluate(&block)
  block.call
end

def reverse_evaluate(&block)
  r2r = Ruby2Ruby.new
  ast = block.to_ast

  pp ast

  ast2 = Sexp.new \
    ast[0],
    Sexp.new(:call, nil, :forward_evaluate, Sexp.new(:arglist)),
    ast[2],
    ( [ ast[3][0] ] + ast[3][1..-1].reverse )

  eval r2r.process(ast2)
end

################################################################

def first
  puts 'first'
end

def second
  puts 'second'
end

def third
  puts 'third'
end

# evaluate statements in a block in normal order

f = lambda {
  first
  second
  third
}

f.call

# first
# second
# third

# evaluate statements in a block in reverse order

reverse_evaluate do
  first
  second
  third
end

# third
# second
# first