Backend Configuration

Company allows for configurable backends, which are just functions that provide completions, many of which are provided in the main company package, but also provided by many third (fourth?) party packages. These backends, then, are in a list which is stored in the company-backends, that company uses to try and find completions. When you get to a moment when you might want to complete things, emacs+company iterate through this list and build a list of expansions. This is pretty straight forward, at least in principle.

Now company is pretty good at making these backends fast, or trying, particularly when the backend might be irrelevant to whatever you're currently editing--except in some special cases--but it means that the order of things in the list matters sometimes. The convention for configuring company backends is to load the module that provides the backend and then push the new backend onto the list. This mostly works fine, but there are some backends that either aren't very fast or have the effect of blocking backends that come later (because they're theoretically applicable to all modes.) These backends to be careful of are: company-yasnippet, company-ispell, and company-dabbrev.

I've never really gotten company-ispell to work (you have to configure a wordlist,) and I've never been a dabbrev user, but I've definitely made the mistake to putting the snippet expansion near the front of the list rather than the end. I've been tweaking things recently, and have settled on the following value for company-backends:

(setq company-backends '(company-capf
                         company-keywords
                         company-semantic
                         company-files
                         company-etags
                         company-elisp
                         company-clang
                         company-irony-c-headers
                         company-irony
                         company-jedi
                         company-cmake
                         company-ispell
                         company-yasnippet))

The main caveat is that everything has to be loaded or have autoloads registered appropriately, particularly for things like jedi (python,) clang, and irony. The "capf" backend is the integration with emacs' default completion-at-point facility, and is the main mechanism by which lap-mode interacts with company, so it's good to keep that at the top.

Make it Fast

I think there's some fear that a completion framework like company could impact the perceived responsiveness of emacs as a whole, and as a result there are a couple of knobs for how to tweak things. Having said that, I've always run things more aggressively, because I like seeing possible completions fast, and I've never seen any real impact on apparent performance or battery utilization. use these settings:

(setq company-tooltip-limit 20)
(setq company-show-numbers t)
(setq company-idle-delay 0)
(setq company-echo-delay 0)

Configure Prompts

To be honest, I mostly like the default popup, but it's nice to be able to look at more completions and spill over to helm when needed. It's a sometimes thing, but it's quite nice:

(use-package helm-company
   :ensure t
   :after (helm company)
   :bind (("C-c C-;" . helm-company))
   :commands (helm-company)
   :init
   (define-key company-mode-map (kbd "C-;") 'helm-company)
   (define-key company-active-map (kbd "C-;") 'helm-company))

Full Configuration

Some of the following is duplicated above, but here's the full configuration that I run with:

(use-package company
  :ensure t
  :delight
  :bind (("C-c ." . company-complete)
         ("C-c C-." . company-complete)
         ("C-c s s" . company-yasnippet)
         :map company-active-map
         ("C-n" . company-select-next)
         ("C-p" . company-select-previous)
         ("C-d" . company-show-doc-buffer)
         ("M-." . company-show-location))
  :init
  (add-hook 'c-mode-common-hook 'company-mode)
  (add-hook 'sgml-mode-hook 'company-mode)
  (add-hook 'emacs-lisp-mode-hook 'company-mode)
  (add-hook 'text-mode-hook 'company-mode)
  (add-hook 'lisp-mode-hook 'company-mode)
  :config
  (eval-after-load 'c-mode
    '(define-key c-mode-map (kbd "[tab]") 'company-complete))

  (setq company-tooltip-limit 20)
  (setq company-show-numbers t)
  (setq company-dabbrev-downcase nil)
  (setq company-idle-delay 0)
  (setq company-echo-delay 0)
  (setq company-ispell-dictionary (f-join tychoish-config-path "aspell-pws"))

  (setq company-backends '(company-capf
                           company-keywords
                           company-semantic
                           company-files
                           company-etags
                           company-elisp
                           company-clang
                           company-irony-c-headers
                           company-irony
                           company-jedi
                           company-cmake
                           company-ispell
                           company-yasnippet))

  (global-company-mode))

(use-package company-quickhelp
  :after company
  :config
  (setq company-quickhelp-idle-delay 0.1)
  (company-quickhelp-mode 1))

(use-package company-irony
  :ensure t
  :after (company irony)
  :commands (company-irony)
  :config
  (add-hook 'irony-mode-hook 'company-irony-setup-begin-commands))

(use-package company-irony-c-headers
  :ensure t
  :commands (company-irony-c-headers)
  :after company-irony)

(use-package company-jedi
  :ensure t
  :commands (company-jedi)
  :after (company python-mode))

(use-package company-statistics
  :ensure t
  :after company
  :config
  (company-statistics-mode))