From: Robert Klemme <shortcutter@...>
Date: 2007-07-25T17:39:33+09:00
Subject: Re: Static variables in Ruby.

2007/7/25, Robert Dober <robert.dober@gmail.com>:
> On 7/24/07, Joel VanderWerf <vjoel@path.berkeley.edu> wrote:
> > Nobuyoshi Nakada wrote:
> > > Hi,
> > >
> > > At Tue, 24 Jul 2007 17:40:57 +0900,
> > > Marcin Tyman wrote in [ruby-talk:261483]:
> > >> Is any way to define static variables over function? As class static
> > >> variables there is possible to define a static class variable as
> > >> @@name_of_var. But I'm not sure if Ruby let define such variables in
> > >> functions (like in C/C++)
> > >
> > >   def foo(x)
> > >     (0..0).instance_eval{x,@x = @x,x}
> > >     x
> > >   end
> > >   3.times{|i|p foo(i)} #=> nil, 1, 2
> > >
> > > The literal object can be Float, Regexp or Bignum, but Range
> > > would be faster a bit.
> > >
> >
> > Interesting. That is because Float, Regexp, and Bignum are instantiated
> > per scope, right?
> >
> > Sometimes a little abstraction leakage can be useful!
> >
> > --
> >        vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407
> >
> >
> I did not believe this,and why should I?
> But it seems to be true ;)
> def foo(x)
>   (0..0).instance_eval{x,@x = @x,x}
>   x
> end
>
> 3.times{ |i| puts foo(i) }
> (0..0).instance_eval{@x=42}
> 3.times{ |i| puts foo(i) }
>
> Run this for a pleasant surprise :)

The reason why it works is this:

>> def f; (0..0).object_id end
=> nil
>> 5.times {p f}
69868270
69868270
69868270
69868270
69868270
=> 5

The Ruby interpreter optimizes this when both ends are constant.

>> def g(a)(0..a).object_id end
=> nil
>> 5.times {p g(1)}
69627090
69626930
69626820
69626680
69626560
=> 5

Kind regards

robert