From: Mikael Brockman <phubuh@...>
Date: 2004-07-07T03:49:51+09:00
Subject: Re: Functional Ruby (Re: Why I don't use Ruby.)

Hal Fulton <hal9000@hypermetrics.com> writes:

> Martin DeMello wrote:
>> Joel VanderWerf <vjoel@path.berkeley.edu> wrote:
>>
>>> Ruby doesn't optimize this tail recursion, but Scheme does. That's
>>> one way Scheme could be said to be more "functional".
>> http://www.ccs.neu.edu/home/matthias/Presentations/ecoop2004.pdf
>> suggests that no language should be called properly OO unless it does
>> tail call optimisation :)
>
> Heh. I hereby pronounce that no language is truly OO unless its name
> starts with an R and ends with a Y.

A noble initiative, but Felleisen's is slightly less arbitrary.  He
shows how a properly OO linked list needs TCO.

  module ProperlyOO
    class Cons
      def initialize (car, cdr)
        @car, @cdr = car, cdr
      end

      def length
        # This requires tail-call optimization.
        1 + cdr.length
      end
    end

    class Empty
      def length
        0
      end
    end
  end

  module ImproperlyOO
    class Cons
      def initialize (car, cdr)
        @car, @cdr = car, cdr
      end

      def length
        # This doesn't require TCO, but it's ugly.
        length = 0
        current = self
        until current.kind_of? Empty
          length += 1
        end
        length
      end
    end

    class Empty
      def length
        0
      end
    end
  end

I guess the iterative version would be better with a method is_empty?,
but I'm just the messenger.

  mikael