# What's a good way of using HAML to generate an HTML nested list from tree
# data?
 
# Example tree data
# Root is the number 12, the children of any node are the factors of the number
NUMBERS = (2..12).to_a
ROOT = NUMBERS.max
CHILDREN_PROC = proc do |number|
  (NUMBERS - [number]).select {|other_number| number % other_number == 0}
end
 
# Expected html
<<-'END_HTML'
<ul>                             
  <li>12 <ul>                           
           <li>2</li>                          
           <li>3</li>                          
           <li>4 <ul>                           
                   <li>2</li>
                 </ul>
           </li>
           <li>6 <ul>
                   <li>2</li>
                   <li>3</li>
                 </ul>
           </li>
         </ul>
  </li>
</ul>
END_HTML
 
# My implementation
# Works, but yielding things like :no_children and explicitly writing
# open/close tags in haml is ugly
def tree_traverse(root, children_proc, &block)
  yield(root)
  children = children_proc.call(root)
  if children.empty?
    yield(:no_children)
  else
    yield(:begin_children)
    children.each do |child|
      tree_traverse(child, children_proc, &block)
    end
    yield(:end_children)
  end
end
 
require 'haml'
ENGINE = Haml::Engine.new <<-'END_HAML'
%ul
  - tree_traverse(ROOT, CHILDREN_PROC) do |node|
    - case node
    - when :begin_children
      <ul>
    - when :end_children
      </ul>
      </li>
    - when :no_children
      </li>
    - else
      == <li>#{node}
END_HAML
puts ENGINE.render