Coercing from Arrays

Question

Suppose I have this simple class:

class Color
  attr_accessor :rgb
  def initialize(ary)
    @rgb = ary
  end
  def +(other)
    other = Color.new(other) unless Color === other
    Color.new(@rgb.zip(other.rgb).map {|p| [p.reduce(:+), 255].min })
  end
end

I know this is a bad way to implement it but this is the shortest way I can think.

c100 = Color.new([100, 100, 100])
c100 + c100         #=> Color(200, 200, 200)
c100 + c100 + c100  #=> Color(255, 255, 255)

It also works if I give an Array as Colors:

c100 + [50, 50, 50] #=> Color(150, 150, 150)

But I can't to this:

[50, 50, 50] + c100 #=> TypeError: can't convert Color into Array

Defining coerce doesn't work. How can I make it working?

Answer 1

It's because the code

[50, 50, 50] + c100

calls the + method on Array, not Color, and that method can't convert a color to an Array.

By contrast,

 c100 + [50, 50, 50]

does call Color's + method.

However, even if you define a conversion method in Color:

class Color
def to_ary
return @rgb
end
end

the Array method will not work as you expect; the result will be the concatenation of the two arrays, since Array's + method concatenates their operands, rather than adding their elements:

irb>[50,50,50]+c100
=> [50,50,50,100,100,100]

Here, the result will be an Array, rather than a Color.

EDIT:

The only way I see of making this work is to alias the + method of Array to handle the special case of receiving a Color as the second operand. However, I will admit that this approach is rather ugly.

class Array
  alias color_plus +
  def +(b)
    if b.is_a?(Color)
      return b+self
    end
    return color_plus(b)
  end
end

Answer 2

Elaborating further on @peter-o answer I came up with this implementation that, although uggly in the sense that it redefines several methods of Array, it pretty much manages to be a good workaround for the expected behaviour, I don't think I would ever fit this in production code but I really liked the challenge... Sorry for diverging on the color subject but I didn't know what the expected behaviour for minus and times would be.

class Array
  alias :former_plus :+
  alias :former_minus :-
  alias :former_times :*

  def +(other)
    former_plus(other)
  rescue TypeError
    apply_through_coercion(other, :+)
  end

  def -(other)
    former_minus(other)
  rescue TypeError
    apply_through_coercion(other, :-)
  end

  def *(other)
    former_times(other)
  rescue TypeError
    apply_through_coercion(other, :*)
  end

  # https://github.com/ruby/ruby/blob/ruby_1_9_3/lib/matrix.rb#L1385
  def apply_through_coercion(obj, oper)
    coercion = obj.coerce(self)
    raise TypeError unless coercion.is_a?(Array) && coercion.length == 2
    coercion[0].public_send(oper, coercion[1])
  rescue
    raise TypeError, "#{obj.inspect} can't be coerced into #{self.class}"
  end
  private :apply_through_coercion
end

One of the chalenges was to make sure the inverted call on the Point#- method would not return unexpected results, hence the @coerced instance variable as a control flag on the object.

class Point
  attr_reader :x, :y

  def initialize(x, y)
    @x, @y, @coerced = x, y, false
  end

  def coerce(other)
    @coerced = true
    [self, other]
  end

  def coerced?; @coerced end

  def +(other)
    other = Point.new(*other) if other.respond_to? :to_ary
    Point.new(@x + other.x, @y + other.y)
  end

  def -(other)
    other = Point.new(*other) if other.respond_to? :to_ary
    if coerced?
      @coerced = false; other + (-self)
    else self + (-other) end
  end

  def -@; Point.new(-@x, -@y) end

  def *(other)
    case other
    when Fixnum then Point.new(@x*other, @y*other)
    when Point  then Point.new(@x*other.x, @y*other.y)
    when Array  then self * Point.new(*other)
    end
  end
end

After all, what this code manages to achieve is adding coerce functionality to the Array class where it didn't exist, explicitly to methods Array#+, Array#- and Array#*.