From: Robert Dober <robert.dober@...>
Date: 2008-04-06T18:01:59+09:00
Subject: Re: The ||= assignment operator

On Sat, Apr 5, 2008 at 10:39 PM, Robert Dober <robert.dober@gmail.com> wrote:
> On Sat, Apr 5, 2008 at 10:02 PM, David A. Black <dblack@rubypal.com> wrote:
>
>
> > Hi --
>  >
>  >
>  >  On Sun, 6 Apr 2008, Robert Dober wrote:
>  >
>  >
>  > > On Sat, Apr 5, 2008 at 7:57 PM, Joel VanderWerf <vjoel@path.berkeley.edu>
>  > wrote:
>  > >
>  > > > David A. Black wrote:
>  > > >
>  > > >
>  > > >
>  > > > > The only time this matters is with hashes that have default values. In
>  > > > > every other case, as far as I know, x = x || y also describes what's
>  > > > > happening. But the expansion which describes *every* case is x || x =
>  > > > > y.
>  > > > >
>  > > > >
>  > > >
>  > > >  One other case (a very similar one) in which it matters is this:
>  > > >
>  > > >  class C
>  > > >    def x; @x || 5; end
>  > > >    def x=(v); puts "assigning @x=#{v}"; @x = v; end
>  > > >  end
>  > > >
>  > > >  c = C.new
>  > > >  c.x ||= 3 # no effect
>  > > >  p c.x     # 5
>  > > >  c.x = 2   # assigning @x=2
>  > > >  p c.x     # 2
>  > > >
>  > > >  --
>  > > >      vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407
>  > > >
>  > > >
>  > > >
>  > > Hmm I am not sure I believe you rushed your example a little bit
>  > >
>  > > c.x = c.x || a
>  > > and
>  > > c.x || c.x = a
>  > > seem to me having the same effect.
>  > >
>  >
>  >  I don't think so:
>  >
>  >  irb(main):017:0> c = C.new
>  >  => #<C:0x87d0c>
>  >  irb(main):018:0> c.x || c.x = 1    # 5 || c.x = 1  =>  5
>  >  => 5
>  >  irb(main):019:0> c.instance_eval { @x }
>
> that is true but that point was difficult to see, and I preferred an
>  example which shows the difference from the "normal" object interface
>  although my code is of course perverted.
and one cannot see the problem through the object interface either :(
In an attempt to make things clear enough I will take Joel's code again:

class << C = Class::new
    def x; @x || 5 end
    def get_x; @x end
    def x= v; @x=v end
end

C.get_x  --> nil
C.x ||= 42
C.get_x --> nil
C.x || C.x = 42
C.get_x --> nil
C.x = C.x || 42
C.get_x --> 5

Hurray I guess I finally got it.
Thx for the enlightenment.

Cheers
Robert

>
> >  => nil
>  >  irb(main):020:0> c.x = c.x || 1    # c.x = 5 || 1  =>  c.x = 5
>  >  assigning @x=5
>  >  => 5
>  >  irb(main):021:0> c.instance_eval { @x }
>  >  => 5
>  >
>  >
>  >
>  >
>  >  David
>  >
>  >  --
>  >  Rails training from David A. Black and Ruby Power and Light:
>  >   ADVANCING WITH RAILS   April 14-17          New York City
>  >   INTRO TO RAILS         June 9-12            Berlin
>  >   ADVANCING WITH RAILS   June 16-19           Berlin
>  >  See http://www.rubypal.com for details and updates!
>  >
>  >
>
>
>
>
>
> --
>  http://ruby-smalltalk.blogspot.com/
>
>  ---
>  Whereof one cannot speak, thereof one must be silent.
>  Ludwig Wittgenstein
>
>



-- 
http://ruby-smalltalk.blogspot.com/

---
Whereof one cannot speak, thereof one must be silent.
Ludwig Wittgenstein