From: Brian Candler <b.candler@...>
Date: 2008-11-03T03:37:52+09:00
Subject: Re: ruby1.9: lazy versions of Enumerator#select and friends?

David A. Black wrote:
> It did have something like this, but not any more. I'm not sure they
> were identical to what you've written, but the basic idea of an
> enumerator that bundled itself with a block did exist in 1.9 for a
> while.

I'd be interested to learn exactly what the dropped feature was.

Perhaps we're talking at cross purposes, but I'm not sure I'm suggesting 
"an enumerator that bundled itself with a block". Rather, I'm suggesting 
that certain methods in Enumerable which used to return an array, 
instead return an Enumerator, so that they can be chained horizontally.

By this I mean:

   a.meth1 { ... }.meth2 { ... }.meth3 { ... }

currently runs "vertically" (meth1 enumerates all of 'a' and creates an 
array; meth2 enumerates this array and creates another array; then meth3 
enumerates this array and creates a final array)

By running "horizontally" I mean that the first element of 'a' is 
processed all the way across; then the second element of 'a'; and so on. 
No intermediate arrays are created.

> I'd strongly go for #1. #2 would require not only massive code
> overhaul, but change of habits, and lots of to_a is unsightly (and you
> wouldn't necessarily want to have to hard-code the class of what you
> want back, anyway [i.e., the 'a' in 'to_a']).

I don't understand the last sentence - a whole bunch of Enumerable 
methods like #map, #select etc are already hard-coded to return an 
Array, so you currently have no choice.

With what I propose you can use any method you like to build the result. 
If you don't want an array, then don't use to_a. You could use 'inject' 
or 'each' or 'each_with_object' to gather the resulting elements in 
whatever way you like: e.g.

  a.meth1 { ... }.meth2 { ...}.each { |x,y| h[x] = y }

Or even, as the original example I posted shows, you can just use the 
results as they arrive and then discard them:

  a.meth1 { ... }.meth2 { ...}.each { |x| puts x }

Now that Enumerators exist, the fact that Enumerable methods are 
hard-coded always to create an Array seems very limiting. There are so 
many other things you might want to create instead.
-- 
Posted via http://www.ruby-forum.com/.