From: "David A. Black" <dblack@...>
Date: 2005-07-13T01:37:23+09:00
Subject: Re: accessing index inside map

Hi --

On Wed, 13 Jul 2005, Brian Candler wrote:

>> Well, apparently we have differing opinions on this.  Personally I find
>> the Enumerator approach simpler as it does not clutter the original's
>> class interface (introducing an optional parameter on each method doubles
>> the number of legal invocations). :-)
>
> My own interpretation of "do the simplest" includes "don't introduce any
> unnecessary abstractions". Arguably, the 'optional' parameter is always
> there; it just happens to default to :each. There are lots of similar cases
> in the Ruby standard library.
>
> The Enumerator stuff is new to me, since I'm still just using 1.8.2.
> However, I find obj.to_enum(:each_with_index) confusing; after all, obj is
> *already* enumerable. Making this more general, you might get
>
>  obj.rename(:each=>:each_with_index).inject(0) {|sum,(a,i)| sum + a*i}
>
> Creating an object just to say "call #each_with_index instead of #each"
> seems wasteful unless you plan to re-use it, given that you could just call
> #each_with_index in the first place.
>
> However, perhaps 'to_enum' could be called something friendlier, e.g.
>
>  obj.using(:each_with_index).inject(0) {|sum,(a,i)| sum + a*i}
>
> In fact, even
>
>  obj.enum(:each_with_index).inject(0) {|sum,(a,i)| sum + a*i}
>
> reads better to me, as it's not stressing the creation of an intermediate
> object. to_foo looks like you are converting obj into something completely
> different, rather than just adding a temporary wrapper.

I wonder whether one might (re)introduce the "iter[ator]" set of
terms, rather than enumerator.  I'm not totally clear on the relation
between them, so I may be off the mark, but I was thinking of things
like:

   obj.iterate_with(:each_with_index).inject ...

a little bit on the model of sort_by.  Then again, this may be getting
dangerously close to the rejected RCR at
http://www.rcrchive.net/rejected.html#rcr50.

I have to admit I haven't totally assimilated the Enumerator
mechanisms, which strike me as very useful and powerful but sort of a
secondary system overlaid on the whole iterator and/or enumerable
thing.  I guess the barrier, for me, is having the separate Enumerator
object, rather than just having it happen in the course of calling a
method.  I guess that's also what's powerful about it :-)

> I'm not familiar with the RCR process.

See http://www.rcrchive.net.


David

-- 
David A. Black
dblack@wobblini.net