maybe.rb

# >> を *
# >>= を bind
# return を self.new
# mplus を +
# mzero を self.zero
#
# に見立てて Maybe モナド書いてみた
# bind に渡す block で Maybe と書きたくないので第二引数に型情報を付加してみた。
class Monad
  def *(m)
    bind { m }
  end

  def join
    bind { |r| r }
  end
end

class Maybe < Monad
  attr_reader :value
  # return
  def initialize(value)
    @value = value
  end

  # >>=
  def bind(&block)
    if just?
      ret = block.call @value, Maybe
      raise '戻り値がモナドじゃないです' unless ret.is_a?(Maybe)
      ret
    else
      Maybe.zero
    end
  end

  def self.zero
    Maybe.new(nil)
  end

  def self.map(&block)
    proc do |r|
      if r.just?
        new block.call(r.value)
      else
        zero
      end
    end
  end

  def +(n)
    if n.just?
      n
    else
      self
    end
  end

  def nothing?
    value.nil?
  end

  def just?
    not nothing?
  end

  def inspect
    if just?
      "Just #{value}"
    else
      "Nothing"
    end
  end
end

Maybe.new(1)
  .bind { |value,monad| monad.new(value * 2) } # => Just 2
(Maybe.new(1) * Maybe.zero)
  .bind { |value,monad| monad.new(value * 2) } # => Nothing
Maybe.map { |n| n*10 }.call Maybe.new(1)       # => Just 10
Maybe.new(1) * Maybe.zero * Maybe.new(2)       # => Nothing
Maybe.zero + Maybe.new(1) + Maybe.zero         # =>  Just 1
Maybe.zero + Maybe.new(1) + Maybe.zero + Maybe.new(3) # => Just 3
Maybe.new(Maybe.new(1)).join                   # =>  Just 1