自作クラスの配列同士で引き算を正しく動作させるには

	class Point
	attr_reader :x, :y
	def initialize(x, y)
	@x = x
	@y = y
	end
	def ==(other)
	@x == other.x && @y == other.y
	end
	def to_s
	"(#{x}, #{y})"
	end
	def inspect
	to_s
	end
	end
	p [Point.new(1, 1), Point.new(2, 2), Point.new(3, 3)] - [Point.new(3, 3), Point.new(1, 1)]
	# => [(1, 1), (2, 2), (3, 3)]

view raw points_subtract_invalid.rb hosted with ❤ by GitHub

	class Point
	attr_reader :x, :y
	def initialize(x, y)
	@x = x
	@y = y
	end
	def ==(other)
	eql?(other)
	end
	def eql?(other)
	@x == other.x && @y == other.y
	end
	def hash
	@x + @y
	end
	def to_s
	"(#{x}, #{y})"
	end
	def inspect
	to_s
	end
	end
	p [Point.new(1, 1), Point.new(2, 2), Point.new(3, 3)] - [Point.new(3, 3), Point.new(1, 1)]
	# => [(2, 2)]

view raw points_subtract_valid.rb hosted with ❤ by GitHub

自作の配列で引き算を行おうとしたら、==のオーバーライドだけでは足りず、eql?とhashのオーバーライドが必要だった。