du.rb

class DirTree < String
    include Enumerable

    def each
        Dir.entries(self).reject{|n| %w[. ..].include? n}.each do |filename|
            yield path = "#{self}/#{filename}"
            DirTree.new(path).each{|n| yield n} if File.directory? path
        end 
    end 
end

class DiskUsage
    def self.[](path)
        DirTree.new(path).map{|n| File.lstat(n).size}.inject(0){|sum, size| sum + size}
    end 
end

p DiskUsage['.']

Based on: http://www.reddit.com/r/haskell/comments/cs54i/how_would_you_write_du_in_haskell/c0uu4ne