From: Joel VanderWerf <vjoel@...>
Date: 2003-05-11T09:11:47+09:00
Subject: [RCR?] Enumerable#each with arguments


I've been using REXML lately and wishing I could do something like this:

   elt = ... # get a REXML::Element somehow
   elt_parts = elt.elements.collect('part') # XPath query

The #collect call fails because it requires 0 arguments. But each can be 
called with an argument, so the following does what I want:

   elt_parts = []
   elt.elements.each('part') { |x| elt_parts << x }

Or I could use #select, but then it's extra work to get the XPath 
functionality.

This leads to a general suggestion: instance methods in Enumerable 
should pass on their arguments to each. To be more specific, here's an 
example that doesn't work now:

   class Foo
     include Enumerable

     def initialize; @stuff = [1,2,3]; end

     def each(n=nil)
       if n
         @stuff.each {|x| yield x if x < n}
       else
         @stuff.each {|x| yield x}
       end
     end
   end

   foo = Foo.new
   foo.each(2) {|x| p x} # ==> 1

   #p foo.collect(2) # ==> should be [1], but wrong number of args

At this point #collect can't be called with arguments. But it's easy to fix:

   class Foo
     def collect(*args)
       rslt = []
       each(*args) {|x| rslt << x}
       rslt
     end
   end

   p foo.collect(2)  # ==> [1]

I could always redefine collect and friends in Enumerable, but why not 
add this behavior in enum.c? Maybe there's a performance cost...?