Defining idempotent conversion functions

point.rb

def Point(*args)
  respond_to_point = ->(arg){ arg.respond_to?(:to_point) }
  respond_to_ary = -> (arg){ arg.respond_to?(:to_ary) }

  case args.first
  when Integer then Point.new(*args)
  when String then Point.new(*args.first.split(':').map(&:to_i))
  when respond_to_point
    args.first.to_point
  when respond_to_ary
    Point.new(*args.first.to_ary)
  else
    raise TypeError, "Cannot convert #{args.inspect} to Point"
  end
end

Point = Struct.new(:x, :y) do
  def inspect
    "#{x}:#{y}"
  end

  def to_point
    self
  end
end

Pair = Struct.new(:a,:b) do
  def to_ary
    [a, b]
  end
end

class Flag
  def initialize(x,y,flag_color)
    @x, @y, @flag_color = x, y, flag_color
  end

  def to_point
    Point.new(@x,@y)
  end
end

p Point([5,7])
p Point(Pair.new(23,32))
p Point(Flag.new(42,24,:red))

Example from the Confident Ruby book