From: Jay McGavren <sgarfunkle@...>
Date: 2007-04-11T04:45:05+09:00
Subject: Procs vs. nested methods and variable scope...

I'm going to be re-using this technique at quite a few places in my
code, so I want to be certain I have it right...

I started with code like this, within a method:

   for playlist in playlists
      with_option_values('find', config) do |terms|
         if (results = playlist.Search(terms, 0))
            results.each {|track| tracks.push(track)}
         end
      end
      with_option_values('visible-find', config) do |terms|
         if (results = playlist.Search(terms, 1))
            results.each {|track| tracks.push(track)}
         end
      end
      #Repeat several more times...
   end

I of course hate repeating myself like that.  I don't want to create a
super-specialized instance or class method, though.  So I tried a
nested method:

   for playlist in playlists
      def search_playlist(playlist, option, config, field_id, tracks)
         with_option_values(option, config) do |terms|
            if (results = playlist.Search(terms, field_id))
               results.each {|track| tracks.push(track)}
            end
         end
      end
      search_playlist(playlist, 'find', config, 0, tracks)
      search_playlist(playlist, 'visible-find', config, 1, tracks)
      #Repeat several more times...
   end

...I wanted to do search_playlist(option, field_id), but I had to add
parameters for the playlist, config, and tracks as well, because they
fall out of scope when the method is called.  Yuck!

So finally, I went with a Proc, because I know a block/Proc stores all
variables that are in scope when it's defined:

   for playlist in playlists
      search_playlist = lambda {|option, field_id|
         with_option_values(option, config) do |terms|
            if (results = playlist.Search(terms, field_id))
               results.each {|track| tracks.push(track)}
            end
         end
      }
      search_playlist.call('find', 0)
      search_playlist.call('visible-find', 1)
      #Repeat several more times, with a lot less typing.
   end

search_playlist falls out of scope when the enclosing method ends, so
I think I'm safe from playlist, tracks, etc. living forever on the
stack, right?  Also, how bad is it that I'm creating the Proc inside a
(not very tight) loop?