From: Brian Candler <B.Candler@...>
Date: 2005-07-13T01:10:19+09:00
Subject: Re: accessing index inside map

> 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.

From the OP's question, then, it's a question of

  [1,2,3,4,5,6].map(:each_with_index) { |x,i| [2,5].include?(i) ? x : x*2 }

versus

  [1,2,3,4,5,6].enum(:each_with_index).map { |x,i| [2,5].include?(i) ? x : x*2 }

I still prefer the first :-) It's a mapping operation, applied directly to
an Array.

> Well, I think a vote is in order.  Did you consider submitting this as
> RCR?

I'm not familiar with the RCR process.

I just thought I'd wait and see what the list members (and Matz in
particular) thought. This idea seems sufficiently obvious that I guess
there's some reason why it has not been implemented already.

Regards,

Brian.