identity_monad.rb

class Identity

  def initialize(obj)
    @obj = obj
  end
  
  def self.wrap(obj)
    Identity.new(obj)
  end
  
  def bind(func)
    func.call(@obj)
  end
  
  def value
    @obj
  end
  
  def ==(o)
    @obj == o.value
  end
  
end

val = 3
iden_val = Identity.wrap(val)

# inc is of type \n -> Identity
inc = -> n { Identity.wrap(n + 1) }
iden = -> n { Identity.wrap(n) }
doub_inc = -> n {inc[n].bind(inc)}

iden_val.bind(inc) == inc[val]
iden_val.bind(iden) == iden_val
iden_val.bind(doub_inc) == iden_val.bind(inc).bind(inc)

maybe_monad.rb

class Maybe

  def initialize(obj)
    @obj = obj
  end
  
  def self.wrap(obj)
    Maybe.new(obj)
  end
  
  def bind(func)
    if @obj.nil?
      nil
    else
      func.call(@obj)
    end
  end
  
  def value
    @obj
  end
  
  def ==(o)
    @obj == o.value
  end
  
end

val = 3
i = Maybe.wrap(val)

inc = -> n { Maybe.wrap(Maybe.wrap(n).bind(-> n {n + 1})) }
iden = -> n { Maybe.wrap(n) }
doub_inc = -> n {inc[n].bind(inc)}

i.bind(inc) == inc[val]
i.bind(iden) == i
i.bind(doub_inc) == i.bind(inc).bind(inc)