From: Robert Klemme <bob.news@...>
Date: 2004-08-10T19:26:16+09:00
Subject: Re: FirstEachLast, an extension to the Enumerable module.


"Alexander Kellett" <ruby-lists@lypanov.net> schrieb im Newsbeitrag
news:20040810095807.GA13593@loki...
> On Tue, Aug 10, 2004 at 06:51:15PM +0900, Robert Klemme wrote:
> > I strongly disagree.  All sorts of problems arise from this approach:
>
> notice my lack of agreement with myself in any case :)
>
> >  - 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)
>
> yes. i'd agree that it belongs to the iteration.
> otherwise i would have coded up the thing i mentioned,
> but... i've never seen an alternate so never bothered.
> could you explain what you mean by delegator in this
> sense? just a usage case example would do. i'd love
> to code this up at some point but never took the time
> to come up with a nice api.

# NOT RECOMMENDED

class IterationState
  attr_accessor :item, :pos

  def method_missing(*a,&b)
    item.send(*a,&b)
  end

  undef :to_s, :to_a, :hash # ... and others
end

module Enumerable
  def each_with_state
    st = IterationState.new

    each_with_index do |e, i|
      st.item = e
      st.pos = i
      yield st
    end

    self
  end

end

ar = %w{foo bar baz bubu}

ar.each_with_state do |x|
  puts "position #{x.pos} elem #{x.item}"
end

> sorry but i dislike these paradigms instensly :/
> the iterator knows,

Not necessarily so:

class RandEnum
  include Enumerable

  def intialize(max=nil) @max=max; end

  def each
    loop { yield rand @max }
  end
end

> so why should i have to duplicate
> code like this. especially in a detect loop this
> is incredibly annoying as in many cases a temporary
> must be used for the returned value as the last statement
> in the loop has to be "old_value = current_value".

You can use inject.

> also having a temporary outside the loop and polluting
> the outer block... is disgusts me...

You can't solve it otherwise because you need state that survives each
iteration step.  That *must* be kept outside the block - either by using a
local or inject.

> maybe i'm just too
> purist but stuff like this *could* be solved so i
> see no reason against doing it :)
>
> thanks for the feedback,

You're welcome.

    robert