Haml and Mustache are friends, not enemies.

See for yourself.

example.yml

---
users:
  - { user: chris }
  - { user: bob }
  - { user: jones }
---

example.haml.mustache

#content
  .left.column
    %h2 Welcome to our site!
  .right.column
    #users

        %p 

This is nothing new. We've covered the mustache command line utility before.

What's new is using Mustache to generate Haml, then generating HTML from that Haml:

$ cat example.yml example.haml.mustache | mustache | haml -s
<div id='content'>
  <div class='left column'>
    <h2>Welcome to our site!</h2>
  </div>
  <div class='right column'>
    <div id='users'>
      <p>chris</p>
      <p>bob</p>
      <p>jones</p>
    </div>
  </div>
</div>

Hamstache.