Update (October 2, 2014): This feature is now available in the ansible-doc package.

In Flycheck I’m using Ansible to setup a testing environment on Travis CI or on a local Vagrant virtual machine. I do not particularly like Ansible, and still much prefer Puppet, but it turned out to be twice as fast as Puppet, and much easier to use for occasional contributors.

One nice feature of Ansible, though, is the ansible-doc command which provides offline documentation for all available Ansible modules. With some lines of Emacs Lisp we can conveniently use this command to view Ansible documentation from within Emacs:

(defvar lunaryorn-ansible-modules nil
  "Cache of all known Ansible modules.")

(defun lunaryorn-ansible-modules ()
  "Get a list of all known Ansible modules."
  (unless lunaryorn-ansible-modules
    (let ((lines (ignore-errors (process-lines "ansible-doc" "--list")))
      (dolist (line lines)
        (push (car (split-string line (rx (one-or-more space)))) modules))
      (setq lunaryorn-ansible-modules (sort modules #'string<))))

(defconst lunaryorn-ansible-doc-buffer " *Ansible Doc*"
  "The Ansible Doc buffer.")

(defun lunaryorn-ansible-doc (module)
  "Show ansible doc for MODULE."
   (list (ido-completing-read "Ansible Module: "
                              nil nil nil nil
                              (thing-at-point 'symbol 'no-properties))))
  (let ((buffer (get-buffer-create lunaryorn-ansible-doc-buffer)))
    (with-current-buffer buffer
      (setq buffer-read-only t)
      (let ((inhibit-read-only t))
        (call-process "ansible-doc" nil t t module))
      (goto-char (point-min)))
    (display-buffer buffer)))

lunaryorn-ansible-doc prompts for the name of an Ansible module, defaulting to the module at point, looks up the documentation with ansible-doc, and displays the resulting buffer. lunaryorn-ansible-modules obtains a list of all modules to provide IDO-based completion in the prompt. Since ansible-doc --list is quite slow, the list of modules is cached in a variable.

Let’s bind this new command to a key:

(eval-after-load 'yaml-mode
  '(define-key yaml-mode-map (kbd "C-c h a") #'lunaryorn-ansible-doc))

Now we can conveniently lookup documentation with C-c h a in a YAML Mode buffer:

Documentation of ansible apt module in Emacs buffer

In case you wonder, why the IDO prompt is vertical, that’s IDO Vertical Mode.