From: Dave Burt <dave@...>
Date: 2006-06-02T09:34:10+09:00
Subject: Re: Adding an Increment Operator?

Dave Howell wrote:
> So, how WOULD I create an .increment operator? I get why I can't put it
> in class Fixnum (well, I think I know why); Fixnums are symbol-ish-ly
> unalterable. 5.increment should fail, for sanity's sake.
> 
> So I have to add .increment to class . . . hmm. I can't add it to
> variables; they're just references. But I want my_var.increment to
> change 'my_var' so that it references the next integer in line.
> 
> Strings have methods that alter strings in place, but a String object is
> a composite object, made up of characters, and !-type methods alter the
> characters of the string without altering the variable's reference.
> Numbers aren't composite objects. Maybe it can't be done? But people are
> constantly amazing me with crazy magical Ruby tricks; it seems so
> improbable that there isn't a way to do this as well . . .
> 
> Somebody else pointed out I could just my_var += 1. Yes, but to me, that
> implies that the "1" is deliberate, and at some other point I might add
> 2 or 5 or something, whereas using "inc" or "++" or whatever says "it
> really doesn't matter if I'm counting by ones, tens, Roman numerals, or
> letters. This variable is just stepping upwards."
> 
> But the main reason I'm asking is not to slightly improve the esthetics
> of a couple lines of code. I'm certain that I'm going to learn something
> interesting from the answer to the question.

As has been pointed out, you can't modify a Fixnum, nor can you ever
assign to self.

I wasn't going to mention the kind of generic delegating container Ross
just wrote about; have a look, but I don't think you'll find a place to
use that where there isn't a better solution.

Another approach might be to encapsulate a counter like so:

class Counter
  attr_accessor :value
  def initialize(i = 0)
    @value = i
  end
  def inc
    @value = @value.succ
  end
end

i = Counter.new
i.value #=> 0
i.inc #=> 1
i.inc #=> 2
i.value = "X"
i.inc #=> "Y"

Or you can do like this:

def counter(i = 0)
  proc { i = i.succ }
end

i = counter
i.call #=> 1
i.call #=> 2
i[] #=> 3

Cheers,
Dave