How to make Ruby endless Range return Infinity-CodePudding

Ruby introduced Endless Range in Ver.2.6 and Beginless in Ver.2.7.

I expect the sizes of endless/beginless or beginless Ranges are always infinite (Float::INFINITY), considering "endless" and beginless are (I interpret as) abstract terms.

However, I realise the size of (only) the endless Range of String is nil (see the last line of the example below), whereas the sizes of (..nil), endless/beginless Numeric Ranges, and even beginless String Ranges are Infinity.

(..nil).size  # => Float::INFINITY
  (..3).size  # => Float::INFINITY
  (3..).size  # => Float::INFINITY
 (..?a).size  # => Float::INFINITY
 (?a..).size  # => nil(!)

In fact, I find that it (= the behaviour for the String class) is the case for an arbitrary user-defined class.

class MyC
  def succ
  end
end

(..MyC.new).size # => Float::INFINITY
(MyC.new..).size # => nil(!)

Why is it so?

How can I make (MyC.new..).size return Float::INFINITY, in the same way as (..MyC.new).size and (..3).size?
I suppose I could redefine Range#size, maybe in combinatin with refine and using, but it seems to me like a dirty hack……

Note all tests above were run with Ruby-3.1.2p20.

CodePudding user response：

Range#size says...

Returns the count of elements in self if both begin and end values are numeric; otherwise, returns nil

Which means (?a..).size is nil as expected, but (..?a).size should also be nil yet it isn't.

Even if one follows the example from the documentation it is still nil.

class Xs
  include Comparable
  attr_accessor :length
  def initialize(n)
    @length = n
  end
  def succ
    Xs.new(@length   1)
  end
  def <=>(other)
    @length <=> other.length
  end
  def to_s
    sprintf "- #{inspect}", @length
  end
  def inspect
    'X' * @length
  end
end

p (..Xs.new(5)).size # => Float::INFINITY
p (Xs.new(5)..).size # => nil(!)

The size documentation and implementation may need to be updated in light of beginless and endless Ranges. This might be a candidate for a bug report.