Maybe Monad

maybe.rb

require 'singleton'
 
class Maybe
  def self.unit(value)
    if value.nil? || (value.respond_to?(:empty?) && value.empty?)
      Nothing.instance
    else
      Just.new(value)
    end
  end
end
 
class Just < Maybe
  attr_reader :value
 
  def initialize(value)
    @value = value
  end
 
  def bind
    yield @value
  end
 
  def empty?
    false
  end
 
  def inspect
    "Just(#{@value.inspect})"
  end
end
 
class Nothing < Maybe
  include Singleton
 
  def bind
    self
  end
 
  def empty?
    true
  end
 
  def inspect
    "Nothing"
  end
end

Example: Math.safe_log

module Math
  def self.safe_log(num, base = Math::E)
    if num < 0 || base < 0
      Nothing
    else
      Just.new(log(num,base))
    end
  end
end

Maybe.unit("hello").bind{|x| Maybe.unit(x*2)}.bind{|x| Maybe.unit(x.length)}

Maybe.unit(nil).bind{|x| Maybe.unit(x*2)}.bind{|x| Maybe.unit(x.length)}

# person.partner.name
Maybe.unit(person).bind{|x| Maybe.unit(x.partner)}.bind{|x| Maybe.unit(x.name)}