Totally impractical and absurd monad syntactic sugar for Ruby, roughly modeled on Scala's for-loop syntax.

monad.rb

require 'continuation'

module Monad
  def -@
    MonadContext.bind(self)
  end

  def wrap(val)
    raise NotImplementedError
  end

  def bind(&b)
    raise NotImplementedError
  end
end

class Maybe
  include Monad
end

class Just < Maybe
  def get
    @val
  end

  def initialize(val)
    @val = val
  end

  def wrap(val)
    Just.new(val)
  end

  def bind(&b)
    b.call(@val)
  end

  def to_s
    "Just(#{@val})"
  end
end

class None < Maybe
  @@inst = self.new

  def wrap(val)
    @@inst
  end

  def bind(&b)
    @@inst
  end

  def to_s
    "None"
  end
end


module Enumerable
  #for some reason we have to copy this here instead of including Monad
  def -@
    MonadContext.bind(self)
  end

  def wrap(val)
    take(1).map{|ignore| val}
  end

  def bind(&b)
    flat_map(&b)
  end
end

def for!(&b)
  MonadContext.new.run(&b)
end

class MonadContext
  def run(&b)
    self.class.stack << self
    begin
      callcc do |cc|
        @cc_stack = [cc]
        @coll_stack = []
        res = b.call
        wrapped = @coll_stack[-1].wrap(res)
        @cc_stack.pop.call(wrapped)
      end
    ensure
      self.class.stack.pop
    end
  end

  def bind(coll)
    @coll_stack << coll
    callcc do |cc|
      res = coll.bind do |el|
        callcc do |cc2|
          @cc_stack << cc2
          cc.call(el)
        end
      end
      @coll_stack.pop
      @cc_stack.pop.call(res)
    end
  end

  def self.bind(coll)
    stack[-1].bind(coll)
  end

  def self.stack
    Thread.current[:MonadContext] ||= []
  end
end

if __FILE__ == $0
  res = for! do
    x =- (1..5)
    y =- (x..5)
    z =- (y % 2 == 0 ? None.new : Just.new(1))
    y + z
  end

  p res
end