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Configuration layers

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1 Introduction

This document is intended as a tutorial for users who are interested in writing their first configuration layer, whether for private use or for contributing upstream. It should help clear up some confusion regarding how layers work and how Spacemacs (and Emacs) loads packages.

2 Nomenclature

Layers and packages. What gives?

A set of Emacs Lisp files that, taken together, provide some feature. Packages may be available on a package repository, such as ELPA or MELPA or on a third-party service provider (such as github) or even locally on the disk.
A collected unit of configuration that can be enabled (or disabled) in Spacemacs. A layer typically brings together one or more packages, as well as the glue configuration code required to make them play well with each other and Spacemacs in general.

Before writing a layer, it is helpful to consider what you are trying to achieve. Is there a package that provides the functionality you are after, and you want to integrate it in Spacemacs? If yes, you should write a layer. Are you trying to implement a new feature that would be useful for the Emacs community at large? In that case, consider whether it wouldn't be more appropriate to write a package first, and then a layer that uses your package.

3 The Emacs loading process

To understand how to best implement a layer, we have to investigate how Emacs loads code.

3.1 Emacs Lisp files

Emacs Lisp files contains code that can be evaluated. When evaluated, the functions, macros and modes defined in that file become available to the current Emacs session. Henceforth, this will be termed as loading a file.

One major problem is to ensure that all the correct files are loaded, and in the proper order. Another issue is to ensure that not too many files are loaded immediately. This causes startup to take too long. Instead, we want to make sure that files are loaded only as needed, and not all at once.

How is this done in Emacs, and how is it done in Spacemacs?

3.1.1 Loading a file

The simplest way to load a file is to call load-file.

(load-file "~/elisp/foo.el")

This is as primitive as it comes. The path must be exact, and it does not have to be in the Emacs load path (we'll get to that later). It will not look for a byte-compiled .elc file. It will simply load exactly what you tell it to.

3.2 Features

A better way to load what you need is to use features. A feature is a symbol that typically has the same name as the file it resides in. Let us say you have the following contents in a file called my-feature.el.

;; Your code goes here ...

(provide 'my-feature)

To have Emacs load this file, call require, as such:

(require 'my-feature)

This checks whether the feature my-feature has already been loaded. If not, it looks for a file called my-feature.el, my-feature.elc or some such. If it finds such a file, it will load it. When the call to provide is evaluated, the feature is added to the list of loaded features, so that subsequent calls to require will do nothing.

This will cause an error if no such file can be found.

The file my-feature.el may very well contain other calls to require, and in fact this is quite a common way to ensure that dependencies are loaded before your code runs.

Package authors should use this technique to make sure that dependencies are loaded before their code runs.

3.2.1 The load path

When loaded using require, Emacs looks for files in its load path. This is nothing more than a list of paths where elisp files can be found, and you can inspect it through SPC h d v load-path in Spacemacs. To add to the load path, simply push to this list, e.g.

(push "/some/path/" load-path)

3.3 Auto-loading

Calling require is nothing more than a glorified way of calling load-file. It solves the problem of ensuring that files are loaded in the correct order, and to some degree it solved the problem of where to find the files on disk but a long list of calls to require at startup would still cause Emacs to take for ever to load.

Emacs uses auto-loading to solve this problem. When a function is registered as auto-loading, an "empty" definition is provided. When that function is called, the file that provides the function is immediately loaded (along with all its required features). Finally, the "empty" function is substituted with the real one and called normally. The end user will see only a slight delay when first calling the function, while subsequent calls to that function (or any other function loaded as part of the same procedure) will be as quick as normal.

To register a function as auto-loadable, we call autoload:

(autoload 'some-function "some-file")

This instructs Emacs that whenever some-function is called, load some-file.el first, and then proceed.

After evaluating the above code, you can try to inspect some-function by doing SPC h d f some-function. It will say it's an auto-loaded function, and that nothing else is known about it until it is loaded. The call to autoload can optionally include more information, such as a doc-string, whether the function can be called interactively, and so on. This provides more information to the end-user without her having to actually load the file first.

Open your elpa directory, go to helm and look at the file helm-autoloads.el. This provides all the auto-loads for all the files in Helm. However, this file is not written by hand. Instead, it is automatically generated from "magic" comments in the source code of Helm. They look like this:

(defun my-function ()
  ;; Source code...

The magic comment ;;;###autoload instructs Emacs that the following definition should be auto-loaded. This automatically generates an appropriate call to autoload.

Things that can be auto-loaded generally involve anything "definable", such as functions, macros, major or minor modes, groups, classes, and so on.

Magic comments also work on other things, such as variable definitions (defvar), but in that case, the definition is just copied verbatim into the auto-loading file. For example, this code will load Helm on startup, long before your file is actually evaluated, probably not what was intended:

(require 'helm)

It is the responsibility of the package authors to ensure that their package can be appropriately auto-loaded, and most packages do this quite well.

Spacemacs makes thorough use of auto-loading. Almost everything in Spacemacs is loaded when needed instead of right away.

3.4 Eval after load

Often, we will want to configure packages after loading them. We may want to set some variables or call some functions. This is trivial with require, because it loads immediately, but it can be tricky with autoloading, because the configuration code must also be deferred.

Emacs offers with-eval-after-load for this purpose. It can be used like this:

(with-eval-after-load 'helm
     ;; Code

This arranges for the relevant code to be executed after Helm is loaded (using either require or an autoload), or if Helm is already loaded, the code is executed immediately.

Since with-eval-after-load is a macro and not a function, its argument does not have to be quoted.

3.5 Use-package

For end users who are trying to put together an efficient Emacs configuration, there is a very useful package called use-package that provides a macro which is also called use-package which does a very good job of streamlining the whole process of loading packages.

The aspiring layer author is recommended to have a look at the use-package documentation. Some examples follow.

(use-package helm)

This simply loads Helm. It is essentially equivalent to (require 'helm).

(use-package helm
  :defer t)

This defers the loading of Helm using the auto-load facility and the auto-load commands provided by the Helm source code. It is, in fact, a no-op.

(use-package helm
  :defer t
  ;; Code to execute before Helm is loaded
  ;; Code to execute after Helm is loaded

This form includes code to execute before and after Helm is loaded. The :init section can be executed immediately, but since Helm is deferred, the :config section is not executed until after loading, if ever. It is essentially equivalent to simply running the :init block, and then adding the :config block in an with-eval-after-load.

(use-package helm
  :commands (helm-find-files helm-M-x))

This creates auto-load references for additional commands, if you find that the package author has been slacking.

(use-package ruby-mode
  :mode "\\.rb\\'")

For packages that provide major modes, you can associate file extensions to that mode by using the :mode keyword. This adds an entry to auto-mode-alist and an auto-load for ruby-mode. Typically this is not required, as ruby-mode should already be auto-loadable, and the package should associate Ruby files with itself already.

Use-package supports heaps of useful keywords. Look at the documentation for more.

4 Anatomy of a layer

A layer is simply a folder somewhere in Spacemacs' layer search path that usually contains these files (listed in loading order).

declare additional layers
the packages list and configuration
all functions used in the layer should be declared here
layer specific configuration
general key bindings

Additionally, for each local package (see the next section), there should be a folder <layer>/local/<package>/ containing the source code for that package. Before initializing that package, Spacemacs will add this folder to the load path for you.

4.1 layers.el

This file is the first file to be loaded and this is the place where addtional layers can be declared.

For instance is layer A depends on some functionality of layer B then in the file layers.el of layer A we can add:

(configuration-layer/declare-layer 'B)

The effect is that B is considered a used layer and will be loaded as if it was added to dotspacemacs-configuration-layers variables.

4.2 packages.el

It contains this list of packages of the layer and the actual configuration for the packages included in the layer.

This file is loaded after layers.el.

It must define a variable called <layer>-packages, which should be a list of all the packages that this layer needs. Some valid package specifications are as follows:

(defconst mylayer-packages
    ;; Get the package from MELPA, ELPA, etc.
    (some-package :location elpa)

    ;; A local package
    (some-package :location local)

    ;; A package recipe
    (some-package :location (recipe
                             :fetcher github
                             :repo "some/repo"))

    ;; An excluded package
    (some-package :excluded t)

The :location attribute specifies where the package may be found. Spacemacs currently supports packages on ELPA compliant repositories, local packages and MELPA recipes (through the Quelpa package). See the MELPA documentation for more information about recipes.

Packages may be excluded by setting the :excluded property to true. This will prevent the package from being installed even if it is used by another layer.

For each included package, you may define one or more of the following functions, which are called in order by Spacemacs to initialize the package.

  1. <layer>/pre-init-<package>
  2. <layer>/init-<package>
  3. <layer>/post-init-<package>

It is the responsibility of these functions to load and configure the package in question. Spacemacs will do nothing other than download the package and place it in the load path for you.

Note: A package will not be installed unless at least one layer defines an init function for it. That is to say, in a certain sense, the init function does mandatory setup while the pre-init and post-init functions do optional setup. This can be used for managing cross-layer dependencies, which we will discuss later.

4.3 funcs.el

It contains all the defined functions used in the layer.

This file is loaded after packages.el and before config.el.

It is good practice to guard the definition of functions to make sure a package is actually used. For instance:

(when (configuration-layer/package-usedp 'my-package)
  (defun spacemacs/my-package-enable () ...)
  (defun spacemacs/my-package-disable () ...))

By guarding these functions we avoid to define them in case the package `my-package` is not used.

4.4 config.el

This file configure the layer like declaring layer variables default values and setup some other variables related to the layer.

This file is loaded after funcs.el.

4.5 keybindings.el

It contains general key bindings.

This is the last file loaded.

The word general here means independent of any package. Since the end user can exclude an arbitrary set of packages, you cannot be sure that, just because your layer includes a package, that package will necessarily be loaded. For this reason, code in these files must be generally safe, regardless of which packages are installed.

More on this in the next section.

5 The Spacemacs loading process

The Spacemacs loading process can be summarized as follows:

  1. Spacemacs goes through all the enabled layers and evaluates their files. The changes introduced by config.el are thus applied, then funcs.el and packages.el are loaded, but nothing happens from packages.el, since these files only define functions and variables.
  2. Spacemacs checks which packages should be downloaded and installed. To be installed, a package must be

    • included by a layer that the user has enabled,
    • not be excluded by any other layer that the user has enabled,
    • not be excluded by the user herself, and
    • there must be at least one <layer>/init-<package> function defined for it.

    Alternatively, if a package is part of the end user's dotspacemacs-additional-packages, it will also be installed.

  3. All packages which should be installed are installed in alphabetical order, package.el built-in Emacs library is in charge of implicit dependencies. Installed packages not following the rules of 2. are removed as well as their dependencies if possible. (This last behavior is optional but default.)
  4. The pre-init, init and post-init functions for each installed package are executed in turn.

It is step four that interests us. It is very important that a package is not installed if no init function is defined for it.

We say that a layer owns a package if it defines an init function for it. A layer does not own a package if it only defines pre-init or post-init functions.

Only one layer may own a package. Since layers are processed in order of specification in the user's dotfile, it is possible for layers to "seize" ownership of a package that was owned by a previously enabled layer.

6 Case study: auto-completion

Spacemacs provides a layer called auto-completion which provides auto-completion features in many modes. It does this using the package company. This layer owns the company package, so it defines a function called auto-completion/init-company.

When a user enables the auto-completion layer, Spacemacs locates it and finds company in the list of packages. Provided that company is not excluded, either by the user or another layer, Spacemacs then locates and runs the init function for company. This function includes a call to use-package that sets up the basic configuration.

However, auto-completion is a two-horse game. By its very nature, it is specific to the major mode in question. It is pointless to expect the auto-completion layer to include configuration for each conceivable major mode, and equally futile to expect each programming language layer (python, ruby, etc.) to fully configure company on their own.

This is solved using the post-init functions. The Python layer, for example, includes the company package and defines a function called python/post-init-company. This function is called after auto-completion/init-company, but it is not called if

  • the auto-completion layer is not enabled, in which case no init function for company will be found, or
  • the company package is excluded either by the user or another layer

As such, python/post-init-company is the only safe place to put configuration related to company in Python mode.

If the Python layer had defined an init function for company, that package would have been installed even if the auto-completion layer had been disabled, which is not what we want.

7 Layer tips and tricks

7.1 Cross-dependencies

Spacemacs provides a couple of additional useful functions you can use to check whether other layers or packages are included.

check if a layer is enabled
check if a package is or will be installed

These are useful in some cases, but usually you can get the desired result just by using post-init functions.

For layers that require another layers to be enabled, use the functions configuration-layer/declare-layer and configuration-layer/declare-layers to ensure that layers are enabled even if the user has not enabled them explicitly. Calls to these functions must go in the config.el file.

7.2 Use-package

In the vast majority of cases, a package init function should do nothing but call to use-package. Again, in the vast majority of cases, all the configuration you need to do should be doable within the :init or :config blocks of such a call.

What goes where? Since :init is executed before load and :config after, these rules of thumb apply.

In :config should be

  • Anything that requires the package to be already loaded.
  • Anything that takes a long time to run, which would ruin startup performance.

The :init block should contain setup for the entry points to the package. This includes keybindings, if the package should be loaded manually by the user, or hooks, if the package should be loaded upon some event. It is not unusual to have both!

7.3 Use-package hooks

Spacemacs includes a macro for adding more code to the :init or :config blocks of a call to use-package, after the fact. This is useful for pre-init or post-init functions to "inject" code into the use-package call of the init function.

(spacemacs|use-package-add-hook helm
  ;; Code
  ;; Code
  ;; Code
  ;; Code

Since a call to use-package may evaluate the :init block immediately, any function that wants to inject code into this block must run before the call to use-package. Further, since this call to use-package typically takes place in the init-<package> function, calls to spacemacs|use-package-add-hook typically happen in the pre-init-<package> functions, and not in post-init-<package>. It is quite safe to do this in pre-init, so that should be the default choice.

7.4 Best practices

If you break any of these rules, you should know what you are doing and have a good reason for doing it.

7.4.1 Package ownership

Each package should be owned by one layer only. The layer that owns the package should define its init function. Other layers should rely on pre-init or post-init functions.

7.4.2 Localize your configuration

Each function can only assume the existence of one package. With some exceptions, the pre-init, init and post-init functions can only configure exactly the package they are defined for. Since the user can exclude an arbitrary set of packages, there is no a priori safe way to assume that another package is included. Use configuration-layer/package-usedp if you must.

This can be very challenging, so please take this as a guideline and not something that is absolute. It is quite possible for the user to break her Spacemacs installation by excluding the wrong packages, and it is not our intention to prevent this at all costs.

7.4.3 Load ordering

In Spacemacs, layers are loaded in order of inclusion in the dotfile, and packages are loaded in alphabetical order. In the rare cases where you make use of this property, you should make sure to document it well. Many will assume that layers can be included in arbitrary order (which is true in most cases), and that packages can be renamed without problems (which is also in most cases).

Preferably, write your layer so that it is independent of load ordering. The pre- and post-init functions are helpful, together with configuration-layer/package-usedp.

7.4.4 No require

Do not use require. If you find yourself using require, you are almost certainly doing something wrong. Packages in Spacemacs should be loaded through auto-loading, and not explicitly by you. Calls to require in package init functions will cause a package to be loaded upon startup. Code in an :init block of use-package should not cause anything to be loaded, either. If you need a require in a :config block, that is a sign that some other package is missing appropriate auto-loads.

7.4.5 Auto-load everything

Defer everything. You should have a very good reason not to defer the loading of a package.

8 How do I … idiomatically?

8.1 Setup auto-completion for a major mode

In your layer's config.el, call spacemacs|defvar-company-backends.

(spacemacs|defvar-company-backends yoyo-mode)

This creates a variable called company-backends-yoyo-mode. In the package init functions, you should push backends to this variable. But of course, only if the auto-completion layer is enabled.

(defconst yoyo-packages '(
                          ;; ...
                          ;; ...

  (when (configuration-layer/package-usedp 'company)
    (defun yoyo/init-some-weird-package ()
      (use-package some-weird-package
        :defer t
        ;; This has to be in init because it's a package entry point
        (push 'some-weird-backend company-backends-yoyo-mode))))

Finally, we must make sure company is started when we enter yoyo-mode, but again only if the auto-completion layer is enabled.

(defconst yoyo-packages '(
                          ;; ...
                          ;; ...

(defun yoyo/init-yoyo-mode ()
  (use-package yoyo-mode
    ;; Some configuration goes here, however nothing relating to company
    ;; since this function may be called even if company is not installed!

(when (configuration-layer/package-usedp 'company)
  (defun yoyo/post-init-yoyo-mode ()
    ;; This makes no reference to `some-weird-package', which may have
    ;; been excluded by the user
    (spacemacs|add-company-hook yoyo-mode)))

Author: Sylvain Benner

Created: 2016-10-03 Mon 00:54