foo.rb

double = -> (e) { e * 2 }

doubleF = map double
doubleF.([1,2,3,4]) #=> [2,4,6,8]
doubleF.([])        #=> []
doubleF.(Just(3))   #=> Just 6
doubleF.(Nothing)   #=> Nothing

kernel.rb

module Kernel
  def map(func = nil, &blk)
    f = func || blk
    -> (e) { e.class.instance_method(:map).bind(e).call(&f) }
  end
end

maybe.rb

module Kernel
  def Just(o)
    Maybe.of(o)
  end
end

class Maybe
  def self.of(value)
    new(value)
  end

  def map(callable)
    Maybe.of(callable.call(@value))
  end

  def to_s
    "Just #{@value}"
  end
  implements Functor
  implements Applicative

  private
  def initialize(value)
    @value = value
  end

  public
  Nothing = new(nil)

  class << Nothing
    def map(callable)
      self
    end

    def to_s
      "Nothing"
    end
  end
end