A Ruby approximation of the Continuation Monad from Haskell, inspired by Dan Piponi's FPComplete tutorial, "The Mother of All Monads": https://www.fpcomplete.com/user/dpiponi/the-mother-of-all-monads

monads.rb

# Inspired by https://www.fpcomplete.com/user/dpiponi/the-mother-of-all-monads, A ruby port of Haskell's continuation monad, using fibers for lazily-evaluated giggles:
#
# For reference, The continuation monad instance definition from haskell:
# instance Monad (Cont r) where
#   return a = Cont ($ a)
#   m >>= k  = Cont $ \c -> runCont m $ \a -> runCont (k a) c

#HI THERE, I AM THE CONTINUATION MONAD
class Cont
  def self.unit(x)
    Cont.new {|nxt| nxt.call(x) }
  end

  def initialize(&block)
    @block = block
  end

  def bind(&block)
    this = self
    Cont.new { |nxt|
      this.run { |unwrapped|
        result = block.call(unwrapped).run
        nxt.call(result)
      }
    }
  end

  def run(&block)
    block ||= lambda {|x| x }
    @block.call(block)
  end

  #DID I MENTION I AM ALSO A FUNCTOR
  def map(&block)
    bind {|x| Cont.unit(block.call(x)) }
  end
end

#HERE ARE SOME THINGS YOU CAN USE ME FOR

#LAZINESS!!!
puts "Doing some sums in the lazy continuation monad"
m = Cont.unit(5).map {|x| puts("actually doing the work now"); x ** 2 }
puts "got our monad right here"
result = m.run
puts "the result is #{result}"

#EARLY TERMINATION!!!
print "x on a good day:"
mx = Cont.unit(11)
my = mx.bind {|x|
    Cont.unit((x + 1).to_s)
}
puts "Hi there x, Can you tell me what is the number after you?"
puts my.run

print "x on a bad day:"
mx = Cont.new { |nxt| "nah mate not interested" }
my = mx.bind {|x|
  puts "This bit never happens"
  Cont.unit((x + 1).to_s)
}
puts "Hi there x, Can you tell me what is the number after you?"
puts my.run

#CHOICE!!!
mx = Cont.new {|nxt| [nxt.call(1), nxt.call(2)] }
my = Cont.new {|nxt| [nxt.call(2), nxt.call(3)] }
mz = mx.bind {|x|
  my.bind {|y|
    Cont.unit(x * y)
  }
}
puts "multiply all the numbers!"
puts mz.run.flatten.inspect

And the output:

[80] pry(main)> edit monads.rb Doing some sums in the lazy continuation monad got our monad right here actually doing the work now the result is 25 x on a good day:Hi there x, Can you tell me what is the number after you? 12 x on a bad day:Hi there x, Can you tell me what is the number after you? nah mate not interested multiply all the numbers! [2, 3, 4, 6]