yin-yang.rb

require 'continuation'
#
# Messing around with mimicking Scheme in Ruby by implementing the Yin/Yang puzzle with Kernel#callcc and a fake 'let' macro
# 
# See also:
#   http://blog.gonzih.me/blog/2013/11/26/yin-yang-callcc-puzzle-in-ruby/, 
#   http://stackoverflow.com/questions/16843853/how-to-implement-let-in-scheme
#

# In Scheme (the original)
#
# (let ((yin ((lambda (cc) (display "@") cc) (call/cc (lambda (c) c))))
#       (yang ((lambda (cc) (display "*") cc) (call/cc (lambda (c) c)))))
#  (yin yang))

# In Ruby without a let (macro-ish method)
#
# lambda do |yin, yang|
#   yin.call(yang)
# end.call(lambda { |cc| print "@"; cc }.call(callcc { |c| c }),
#          lambda { |cc| print "*"; cc }.call(callcc { |c| c }))

# my let method with some funky metaprogramming
def let(bindings, &blk)
  Class.new do
    def initialize(bindings)
      bindings.each do |name, value|
        instance_variable_set(:"@#{name}", value)
        self.class.send(:attr_reader, name.to_sym)
      end
    end

    define_method :eval, &blk
  end.new(bindings).eval
end

# the result is pretty comparable to the scheme version
let yin: -> (cc) { print "@"; cc }.(callcc { |c| c }),
    yang: -> (cc) { print "*"; cc }.(callcc { |c| c }) do
  yin.(yang)
end