ScalaのOptionをRubyで説明してみる

option.rb

#! /bin/env ruby

class Option
end

class Some < Option
  include Enumerable

  def initialize(a)
    @a = a
  end

  def each
    yield @a
  end

  def get_or_else(b)
    @a
  end

  def or_else(b)
    self
  end

  def map(&block)
    self.class.new( yield @a )
  end

  def flat_map(&block)
    yield @a
  end

  def to_a
    [@a]
  end
end

class None < Option
  include Enumerable
  
  def each
    # do nothing
  end

  def get_or_else(b)
    b
  end

  def or_else(b)
    b
  end

  def map(&block)
    self
  end

  def flat_map(&block)
    self
  end

  def to_a
    []
  end
end

class Object
  def some
    Some.new(self)
  end
end

def none
  None.new
end

if __FILE__ == $0
  # or_else, get_or_elseの例
  # 名前欄にはfirst_nameとlast_nameどちらか設定されているほうを表示する
  # どちらも設定されていなければ"名無しさん"と表示する
  User = Struct.new(:first_name, :last_name) do
    def display_name
      self.first_name.or_else(self.last_name).get_or_else("名無しさん")

      # Rubyだとこっちのほうが簡単
      # self.first_name || self.last_name || "名無しさん"
    end
  end
  taro = User.new("Taro".some, "Yamada".some)
  jiro = User.new(none, "Yamada".some)
  saburo = User.new("Saburo".some, none)
  shiro = User.new(none, none)

  print "TARO: ";   p taro.display_name
  print "JIRO: ";   p jiro.display_name
  print "SABURO: "; p saburo.display_name
  print "SHIRO: ";  p shiro.display_name

  # flat_mapを使った計算の連鎖の例
  a = 1.some
  b = 2.some
  c = 3.some
  ans1 = a.flat_map {|i|
    b.flat_map {|j|
      c.map {|k|
        i + j + k
      }
    }
  }
  print "ans1 is "
  p ans1
  p ans1.get_or_else(-1)

  # 途中で計算に失敗するとそこで打ち切られる（ぬるぽ安全）
  a = 1.some
  b = none
  c = 3.some
  ans2 = a.flat_map {|i|
    b.flat_map {|j|
      c.map {|k|
        i + j + k
      }
    }
  }
  print "ans2 is "
  p ans2
  p ans2.get_or_else(-1)

  # Nullを含むカラムの扱い例
  Stock = Struct.new(:name, :category)
  stocks = [
    Stock.new("stock1", "衣類".some),
    Stock.new("stock2", "靴".some),
    Stock.new("stock3", none),
    Stock.new("stock4", "衣類".some),
  ]
  category_names = stocks.flat_map{|stock| stock.category.to_a }.uniq.sort.join(",")
  print "categoryの一覧は "
  p category_names
end