From: Robert Klemme <bob.news@...>
Date: 2004-08-10T18:51:15+09:00
Subject: Re: FirstEachLast, an extension to the Enumerable module.


"Alexander Kellett" <ruby-lists@lypanov.net> schrieb im Newsbeitrag
news:20040810090629.GA13191@loki...
> On Tue, Aug 10, 2004 at 03:46:17PM +0900, Robert Klemme wrote:
> > Hm, sounds to me like it was not general enough to include it in
> > Enumerable.  Before I see that method I'd prefer to have size and
empty?
> > in Enumerable.  Just my 0.02 EUR...
>
> personally the idea of having a method taking 4 procs
> in the stdlib is just a bit strange.

Agreed.

> i'd prefer to see
> a generic set of methods that are somehow mixedin to
> the values that a Enumerable yields, therefore allowing
> things like val.last? or first? or even a generic
> val.enum_index to replace the each_with_index rubbish.
> i've no idea how to solve this neatly unfortunately as
> extend'ing the objects that are yield'ed of course will
> have sideeffects... though most of the side effects that
> will cause problems shouldn't happen at all, e.g people
> calling is_a? a lot will have problems...

I strongly disagree.  All sorts of problems arise from this approach:

 - permanent modification of elements' types
 - method name collisions
 - member state collisions
 - thread problems (1 instance used during two parallel iterations)

The index clearly belongs to the *iteration* and not the elements.
Possible solutions:

 - yield not the instance but an iteration state that has the current item
as member
 - use delegator (this avoids permanent changes but has still method
collision problems)

> i do this
> and yet i don't have an exception to such an extension...
> but this:
>
>    blah.each {
>       |element|
>       pos = blah.index element
>       puts blah[pos-1]
>       puts element
>    }
>
> is soooooo uggllyyy... not to mention inefficient...
> i'd just loooveeee to see a element.previous_in_enum...

First of all you don't cope with the first element properly.  Then, there
are simple and efficient solutions available.  To name some:

# for Arrays
blah.each_with_index {
   |element, pos|
   puts blah[pos-1] if pos != 0
   puts element
}

NOTHING = Object.new
last = NOTHING
blah.each {
   |element|
   puts last if NOTHING != last
   puts element
   last = element
}

NOTHING = Object.new
blah.inject(NOTHING) {
  |last, element|
  puts last if NOTHING != last
  puts element
  element
}

NOTHING = Object.new
blah.inject(NOTHING) {
  |last, element|
  process last if NOTHING != last
  element
}
process last if NOTHING != last


etc. depending on what you want to do.

Regards

    robert