Simple example to understand monads for Rubyists. See http://en.wikipedia.org/wiki/Monad_(functional_programming)#Formal_definition.

monad.rb

class Monad
  attr_reader :value

  def bind(f)
    raise "not implemented"
  end

  def self.return(v)
    self.new v
  end                                                                                                                                                                                                                                       

  def initialize(value)
    @value = value
  end

  def self.construct(monad, *methods)
    if methods.empty?
      monad                                                                                                                                                                                                                                 
    else
      construct(monad.bind(methods.first),
                *methods[1..-1])
    end
  end
end

class ReachedLimit < Monad
  def self.bind(f)
    ReachedLimit
  end
end

class Result < Monad
  def bind(f)
    result = f.call self.value
    result > 1000 ? ReachedLimit : Result.new(result)
  end
end

def twice(a)
  2 * a
end

def plus100(a)
  100 + a
end

def times4(a)
  4 * a
end

def times200(a)
  200 * a
end