emacs-ycmd is a client for ycmd, the code completion system. It takes care of managing a ycmd server and fetching completions from that server.

emacs-ycmd comprises a core set of functionality for communicating with ycmd as well as integration with the Emacs completion framework company-mode.

A lot of the concepts behind emacs-ycmd are actually concepts from ycmd itself, so if you feel lost you might read the ycmd documentation and/or the the original YouCompleteMe documentation.

Important: The ycmd package itself doesn't provide a real UI for selecting and inserting completions into your files. For that you need to use company-ycmd or another "completion framework".

First make sure that ycmd is installed on your system. See the ycmd instructions for more details.

To use ycmd-mode in all supported modes, add the following to your emacs config:

(require 'ycmd)
(add-hook 'after-init-hook #'global-ycmd-mode)

Or add ycmd-mode to a specific supported mode:

(require 'ycmd)
(add-hook 'c++-mode-hook 'ycmd-mode)

Use the variable ycmd-server-command to specify how to run the server. It will typically be something like:

(set-variable 'ycmd-server-command '("python" "/path/to/ycmd/package/"))

NB: We do not do filename expansion on the elements of ycmd-server-command. As a result, paths using "~" to represent the home directory will not work properly; you need to expand them yourself. For example:

(set-variable 'ycmd-server-command `("python" ,(file-truename "~/.emacs.d/ycmd/ycmd/")))

If you've got a global ycmd configuration, specify that in your emacs configuration by setting ycmd-global-config:

(set-variable 'ycmd-global-config "/path/to/global_config.py")

Spacemacs users: Note that if you don't set ycmd-global-config, spacemacs will set it for you. This is not always what you want! See the spacemacs ycmd documentation for more info.

If you've got project-specific ycmd configurations (almost certainly called .ycm_extra_conf.py), and if you want them automatically loaded by ycmd as needed (which you probably do), then you can whitelist them by adding entries to ycmd-extra-conf-whitelist. For example, this will allow automatic loading of all .ycm_extra_conf.py files anywhere under ~/my_projects

(set-variable 'ycmd-extra-conf-whitelist '("~/my_projects/*"))

Alternatively, you can set ycmd-extra-conf-handler to control how ycmd.el deals with non-whitelisted extra configs. By default this is set to 'ask, meaning it will ask the user each time one is encountered. The other options are 'ignore, in which case the extra config will be ignored, and 'load, in which case the extra config will be loaded.

Now a ycmd server will be automatically launched whenever it's needed. Generally, this means whenever you visit a file with a supported major mode. You should not normally need to manually start or stop a ycmd server.

With a server running, you can now get completions for a point in a file using ycmd-get-completions. This doesn't actually insert the completions; it just fetches them from the server. It's not even an interactive function, so you can't really call it while editing. If you just want to see the possible completions at a point, you can try ycmd-display-completions which will dump a raw completion struct into a buffer. This is more of a debugging tool than anything.

It is recommended to use company-mode for completion, however there is basic support for Emacs' built-in completion mechanism.

(defun ycmd-setup-completion-at-point-function ()
  "Setup `completion-at-point-functions' for `ycmd-mode'."
  (add-hook 'completion-at-point-functions
            #'ycmd-complete-at-point nil :local))

(add-hook 'ycmd-mode-hook #'ycmd-setup-completion-at-point-function)

More likely, you'll want to use a completion framework like company-mode to manage the completions for you. Here's how to do that:

(require 'company-ycmd)
(company-ycmd-setup)

After this you can use your standard company-mode keybindings to do completion.

There have been some reports that ycmd.el doesn't work when Python's output is buffered. See, for example, issue #104. This is because we rely on the ycmd server printing out its host and port information in a timely (i.e. unbuffered) manner. We will almost certainly update the defaults for ycmd.el to force unbuffered output.

In any event, if you are facing problems with ycmd not starting and/or hanging Emacs, try adding -u to your ycmd-server-command. For example:

(set-variable 'ycmd-server-command '("c:/path/to/python.exe" "-u" "c:/path/to/ycmd"))

flycheck-ycmd.el allows you to use ycmd as a backend for flycheck. With this enabled, whenever ycmd parses a file the results will be passed to flycheck for display. This is a really nice way to get quick feedback on problems in your code.

The simple way to enable flycheck integration is to use flycheck-ycmd-setup:

(require 'flycheck-ycmd)
(flycheck-ycmd-setup)

This will make sure that flycheck sees the parse results, and that the flycheck-ycmd backend is enabled.

If for some reason you want to do this manually, the instructions are like this:

(require 'flycheck-ycmd)

;; Make sure the flycheck cache sees the parse results
(add-hook 'ycmd-file-parse-result-hook 'flycheck-ycmd--cache-parse-results)

;; Add the ycmd checker to the list of available checkers
(add-to-list 'flycheck-checkers 'ycmd)

If you use flycheck-ycmd-setup or otherwise put ycmd at the front of flycheck-checkers, flycheck will use the ycmd checker for every buffer in ycmd-mode. This may not be what you want. For example, even though ycmd supports completion (and, thus, flycheck) for Python, you may wish to use pyflakes for flychecking Python code.

To disable ycmd-based flychecking for specific modes, you can modify the flycheck-disabled-checkers list in your mode hook. For example:

(add-hook 'python-mode-hook (lambda () (add-to-list 'flycheck-disabled-checkers 'ycmd)))

With this, the ycmd checker will be ignored in python-mode. Since flycheck-disabled-checkers is buffer-local, the ycmd-based checker will still be available for other modes.

In some cases you may see that company and flycheck interfere with one another. You can end up with strange completion artifacts in your buffers. This mostly seems to happen when you run emacs in "terminal mode", i.e. with emacs -nw.

The short answer for how to deal with this is:

(setq flycheck-indication-mode nil)

The slightly longer and probably better answer is:

(when (not (display-graphic-p))
  (setq flycheck-indication-mode nil))

For a full explanation see the emacs-ycmd defect related to this as well as the root flycheck issue.

ycmd-eldoc adds eldoc support for ycmd-mode buffers.

(require 'ycmd-eldoc)
(add-hook 'ycmd-mode-hook 'ycmd-eldoc-setup)

Note: eldoc messages will only be shown for functions which are retrieved via semantic completion.

emacs-ycmd reports found errors through emacs buttons; to integrate those with next-error prepend something like (require 'ycmd-next-error) before require'ing ycmd (after adding the contrib directory to your load-path).

In some common configurations emacs-ycmd can produce lots of messages, and some people find these noisy and distracting. If you're seeing a lot of messages like Contacting host: 127.0.0.1:NNNNN and you'd like to quiet them, set url-show-status to nil. This can effect non-ycmd-related buffers, so consider using buffer-local settings if this worries you.

You might also see a flurry of messages like this:

REQUEST [error] Error (error) while connecting to http://127.0.0.1:38987/completions.
REQUEST [error] Error (error) while connecting to http://127.0.0.1:38987/event_notification. [26 times]

These almost never indicate something you need to be concerned about. To quiet them, you can set request-message-level to -1.

See issue #173 for the initial discussion of this topic.

emacs-ycmd comes with a number of tests that you can run. This is mostly useful for developers. They are built with ert, so you can run them using any technique that ert provides. For example:

(require 'ycmd-test)
(ert-run-tests-interactively "ycmd-test")

It is also possible to run the tests on the command-line with the Makefile provided in this repository. Before running test, you need to install the Cask in order to be able to install the package dependencies.

You can do this by running

make deps

The other thing that is required is to have the ycmd folder right next to emacs-ycmd (../ycmd).

To run the tests:

make test

It is also possible to have the ycmd server at a different location. In that case the path needs to be passed to the make command explicitly:

make YCMDPATH='/path/to/ycmd/ycmd' test

Make sure that you provide the path to the ycmd module and not the ycmd root directory.