I have got this haml/mustache template:
  {{#data}}   ok   {{#items}}     {{#item}}       %b ID: {{id}}     {{/item}}   {{/items}} {{/data}} 
  And I have got Illegal nesting: nesting within plain text is illegal Error.
  I render it in Sinatra
  Mustache.render(haml(:index), hash) 
       I'm not sure about rendering with Sinatra, but with this command:
  cat example.yml foo.haml.mustache | mustache | haml -e 
  this data file example.yml
  --- data:   - items:     - item:       - id: 1       - id: 2       - id: 3 ---     
  and template (foo.haml.mustache ):
  {{#data}} #ok {{#items}} {{#item}}   %b ID: {{id}} {{/item}} {{/items}} {{/data}} 
  I get following result:
     ID: 1   ID: 2   ID: 3 
 
  Pls pay attention to indentation level in *.mustache file. Hope this help you.