From: Kristof Bastiaensen <kristof@...>
Date: 2004-05-13T20:18:51+09:00
Subject: Re: in search of a compelling reason to use ruby....

On Thu, 13 May 2004 10:22:42 +0200, Robert Klemme wrote:
> 
> Yeah, that's true, although you could do
> 
> a.grep( Criterion.create {|x| x.length > 2} ) {|x| "(#{x})"}
> 
> or even if you define Criterion as method in Kernel:
> 
> a.grep( Criterion {|x| x.length > 2} ) {|x| "(#{x})"}
> 
>> Clever, though, and probably useful in some situations (e.g. where
> you're
>> doing *lots* of filtering and mapping :)
> 
> Exactly.  I thought of a situation where you use this idiom often and you
> have Criterion as a lib module that you use over and over again.
> 

Or another idea:

require "enumerator"
module Enumerable
  def filter(block)
    each do |i|
      yield i if block.call(i)
    end
  end
  def enum_filter(&block)
    Enumerator.new(self, :filter, block)
  end
end

(3..20).enum_filter{ |x| x % 2 == 0 }.collect
=> [4, 6, 8, 10, 12, 14, 16, 18, 20]

Kristof