for automating Clojure projects without setting your hair on fire

Table of Contents


Leiningen Plugins

Leiningen tasks are simply functions named $TASK in a leiningen.$TASK namespace. So writing a Leiningen plugin is just a matter of creating a project that contains such a function, but much of this documentation applies equally to the tasks that ship with Leiningen itself.

Using the plugin is a matter of declaring it in the :plugins entry of the project map. If a plugin is a matter of user convenience rather than a requirement for running the project, users should place the plugin declaration in the :user profile in ~/.lein/profiles.clj instead of directly in the project.clj file.

Not Writing a Plugin (无为)

The first thing to do when writing a plugin is to try to accomplish what you’re doing without a plugin.

Early on in the days of Leiningen many plugins were written which did nothing but provide a short command to run a specific function using eval-in-project. Once Leiningen added the support for partially-applied aliases these became largely redundant, because you can add an alias to the run task:

:aliases {"mytest" ["run" "-m" "mylib.test/go"]}

Not only does this allow lein mytest to run the mylib.test/go function inside the context of your project, it also passes additional arguments (such as in the case of lein run mytest :integration) on to the function specified. However, for some plugins this wasn’t enough as they needed access to values in the project map. For instance, a test runner would need to know the value of :test-paths to know which directory to scan for tests.

As of Leiningen 2.4.0 it’s possible to get this data from an alias, removing the need for a plugin.

:aliases {"mytest" ["run" "-m" "mylib.test/go" :project/test-paths]}

This will load the :test-paths value from the project map and pass a string representation of it as the first argument to the function specified in the alias, followed by any command-line arguments given to the mytest alias. It’s up to the function to call read-string on that argument.

However, if you need to call other Leiningen functions or have no need to run anything inside the context of the project’s own process, making a plugin might be the right choice if one doesn’t exist already.

Writing a Plugin

Start by generating a new project with lein new plugin myplugin, and edit the myplugin defn in the leiningen.myplugin namespace. You’ll notice the project.clj file has :eval-in-leiningen true, which causes all tasks to operate inside the leiningen process rather than starting a subprocess to isolate the project’s code. Plugins need not declare a dependency on Clojure itself; in fact all of Leiningen’s own dependencies will be available.

See the lein-pprint directory in the Leiningen source for a sample of a very simple plugin.

When emitting output, please use leiningen.core.main/info, leiningen.core.main/warn, and leiningen.core.main/debug rather than println since these will respect the user’s output settings.

Local development

When you’re ready to test your plugin in a separate project you can include it via the following (example a plugin named sample-plugin):

lein install
Created ~/sample-plugin/target/sample-plugin-0.1.0-SNAPSHOT.jar
Wrote ~/sample-plugin/pom.xml
Installed jar and pom into local repo.

This will build a jar using the :version listed in the plugin’s project.clj file (see above for example project.clj) and install it into your local m2 repository (~/.m2/repository)

After this step completes you can now list your plugin in your separate project with the version outputted from above. This example would look like this:

:plugins [[sample-plugin "0.1.0-SNAPSHOT"]]

During local development, having to re-run lein install in your plugin project and then switch to a test project can be very cumbersome. In order to avoid this annoyance, you can do the following:

  1. If you haven’t done it yet, run lein install in the plugin’s project directory.
  2. Just to make sure, run lein help <plugin-name> in your test project directory. A help message for your plugin should be displayed now. Or an exception originating in your plugin.
  3. Add the path to the src directory of your plugin to the file .lein-classpath in your test project directory. Probably you’ll have to create that file.
  4. If your plugin depends on another library that you are also working on, then that needs to be added to .lein-classpath with the classpath separator, either : for unix, or ; for Windows. The same goes for your plugin’s other direct dependencies. Run lein classpath in order to get an idea how the contents of .lein-classpath are supposed to look.
  5. Remove the entry for your plugin from the test project’s project.clj. Otherwise it would override what you’ve added to .lein-classpath, because Leiningen loads those things before it loads plugins.

Task Arguments

The first argument to your task function should be the current project. It will be a map which is based on the project.clj file, but it also has :name, :group, :version, and :root keys added in, among other things. Try using the lein-pprint plugin to see what project maps look like; you can invoke the pprint task to examine any project or combination of profiles.

If you want your task to take parameters from the command-line invocation, you can make the function take more than one argument. In order to underscore the fact that tasks are just Clojure functions, arguments which act as flags are usually accepted as :keywords rather than unixy traditional --dashed syntax. Note that all arguments are still passed in as strings; it’s up to your function to call read-string on the arguments if you want keywords, symbols, integers, etc. Keep this in mind when calling other tasks as functions too.

Most tasks may only be run in the context of a project. If your task can be run outside a project directory, add ^:no-project-needed as metadata to your task defn to indicate so. Your task must still accept a project as its first argument, but it will be allowed to be nil. Leiningen will still pass you the project as first argument if lein is called from within a project. If called outside of a project, lein will send in profile information from $HOME/.lein/profiles.clj and similar sources as a map similar to a project map. Other tools using the leiningen-core library (IDE integration, etc) may decide to just pass in nil. To distinguish between a project and non-project, check for the :root key. If it’s set, then you are in a project, otherwise you are not.

Documentation and subtasks

The lein help task uses docstrings. A namespace-level docstring will be used as the short summary if present; if not then it will take the first line of your function’s docstring. Try to keep the summary under 68 characters for formatting purposes. The full docstring can of course be much longer but should still be wrapped at 80 columns. The function’s arglists will also be shown, so pick argument names that are clear and descriptive. If you set :help-arglists in the function’s metadata, it will be used instead for those cases where alternate arities exist that aren’t intended to be exposed to the user. Be sure to explain all these arguments in the docstring. Note that all your arguments will be strings, so it’s up to you to call read-string on them if you want keywords, numbers, or symbols.

Often more complicated tasks get divided up into subtasks. Placing :subtasks metadata on a task defn which contains a vector of subtask vars will allow lein help $TASK_CONTAINING_SUBTASKS to list them. This list of subtasks will show the first line of the docstring for each subtask. The full help for a subtask can be viewed via lein help $TASK_CONTAINING_SUBTASKS $SUBTASK.

Note that Leiningen doesn’t have a mechanism for automatically invoking subtasks. You’ll have to do that yourself in the main task. A dumb implementation of it all might look like this:

(defn my-task
  "Automatically write all the project's code."
  {:subtasks [#'my-subtask-0 #'my-subtask-1]}
  [project & [sub-name]]
  (case sub-name
    "my-subtask-0" (my-subtask-0 project args)
    "my-subtask-1" (my-subtask-1 project args)
    nil            :not-implemented-yet
    (leiningen.core.main/warn "Unknown task.")))

Leiningen will intercept calls to lein $MYTASK help by default and turn them into lein help $MYTASK. If your task provides its own help subtask you can add ^:pass-through-help metadata to your task defn to opt-out of this behaviour.

Code Evaluation

Plugin functions run inside Leiningen’s process, so they have access to all the existing Leiningen functions. The public API of Leiningen should be considered all public functions inside the leiningen.core.* namespaces not labeled with ^:internal metadata as well as each individual task functions. Other non-task functions in task namespaces should be considered internal and may change inside point releases.

Evaluating In Project Context

Many tasks need to execute code inside the context of the project itself. The leiningen.core.eval/eval-in-project function is used for this purpose. It accepts a project argument as well as a form to evaluate, and the final (optional) argument is another form called init that is evaluated up-front before the main form. This may be used to require a namespace earlier in order to avoid the Gilardi Scenario.

Inside the eval-in-project call the project’s own classpath will be active and Leiningen’s own internals and plugins will not be available.

You can modify the project map before you pass it into eval-in-project. However, it’s recommended that you make your modifications by merging a profile in so users can override your changes if necessary. Use leiningen.core.project/merge-profiles to make your changes:

(def swank-profile {:dependencies [['swank-clojure "1.4.3"]]})

(defn swank
  "Launch swank server for Emacs to connect. Optionally takes PORT and HOST."
  [project port host & opts]
    (let [profile (or (:swank (:profiles project)) swank-profile)
          project (project/merge-profiles project [profile])]
      (eval-in-project project
                       `(swank.core/-main ~@opts)
                       '(require 'swank.core))))

The code in the swank-clojure dependency is needed inside the project, so it’s declared in its own profile map and merged in. However, we defer to the :swank profile in the project map if it’s present so that the user can pick their own version of the dependency if they like rather than relying on the hard-coded profile in the plugin.

Note that the snippet above is not a good example of a plugin since it simply wraps eval-in-project and merge-profiles. If that is all you want, you can do it without implementing a plugin; just define an alias that uses the with-profiles and run tasks to call the function you need.

Before eval-in-project is invoked, Leiningen must “prep” a project, usually by ensuring that all Java code and all necessary Clojure code has been AOT compiled to bytecode. This is done by running all the tasks in the :prep-tasks key of the project, which defaults to ["javac" "compile"]. If your plugin requires another kind of prepping, (for instance, compiling protocol buffers) you can instruct users to add another entry to :prep-tasks. Note that this task will be invoked for every eval-in-project, so take care that it runs quickly if nothing has changed since the last run.

Other Plugin Contents

Plugins are primarily about providing tasks, but they can also contain profiles, hooks, middleware, wagons (dependency transport methods), and vcs methods.


If there is configuration that is likely to be used by many projects using your plugin, yet for some reason you can’t make that configuration active by default, you can include profiles inside your plugin.

Create a file called resources/myplugin/profiles.clj in your plugin that contains a map:

{:default {:x "y and z"}
 :extra {:other "settings"}}

Each value here is a profile that your users can merge into their project. You can do this explicitly on a per-invocation basis using with-profile:

$ lein with-profile plugin.myplugin/extra test

Users can also have profiles activated automatically by changing the :default profile:

:profiles {:default [:base :system :user :provided :dev :plugin.myplugin/default]
           :other {...}}

Everything in the :default profile is active for all non-with-profile task invocations except for those which produce downstream artifacts, like jar, uberjar, and pom.


Note: Leiningen supports loading hooks from plugins; however this mechanism is extremely error-prone and difficult to debug. It should be considered deprecated as of 2.8.0 onward and will continue to work until version 3.0 but is strongly advised against.

You can modify the behaviour of built-in Leiningen tasks to a degree using hooks. Hook functionality is provided by the Robert Hooke library, which is included with Leiningen.

Inspired by clojure.test’s fixtures functionality, hooks are functions which wrap other functions (often tasks) and may alter their behaviour by binding other vars, altering the return value, only running the function conditionally, etc. The add-hook function takes a var of the task it’s meant to apply to and a function to perform the wrapping:

(ns lein-integration.plugin
  (:require [robert.hooke]

(defn add-test-var-println [f & args]
  `(binding [~'clojure.test/assert-expr
             (fn [msg# form#]
               (println "Asserting" form#)
               ((.getRawRoot #'clojure.test/assert-expr) msg# form#))]
     ~(apply f args)))

;; Place the body of the activate function at the top-level for
;; compatibility with Leiningen 1.x
(defn activate []
  (robert.hooke/add-hook #'leiningen.test/form-for-testing-namespaces

Hooks compose, so be aware that your hook may be running inside another hook. See the documentation for Hooke for more details. Note that calls to add-hook should use the var for both the first and second argument so that hooks can be loaded repeatedly without re-adding the hook. This is because in Clojure bare functions cannot be compared for equality, but vars can.

If you want your hooks to be loaded automatically when other projects include your plugin, activate them in a function called plugin-name.plugin/hooks. So in the example above the plugin is called lein-integration, and the function lein-integration.plugin/hooks is automatically called to activate hooks when the lein-integration plugin is loaded.

Hooks can also be loaded manually by setting the :hooks key in project.clj to a seq of vars to call to activate your hooks. For backward compatibility, you can also specify namespaces instead of vars in :hooks, and the activate function in that namespace will be called. Note: automatic hooks are activated before manually specified hooks.

Project Middleware

Project middleware is just a function that is called on a project map returning a new project map. Middleware gives a plugin the power to do any kind of transformation on the project map. However, problems with middleware can be difficult to debug due to their flexibility and opaqueness. If you can do what you need using profiles inside your plugins instead, that is a much more declarative, introspectable way to do things which will save a lot of headache down the line.

Projects use middleware by adding :middleware as a vector of var names into their project.clj:

  :middleware [leiningen.inject/middleware]

Also note that the currently active middleware depends on which profiles are active. This means we need to reapply the middleware functions to the project map whenever the active profiles change. We accomplish this by storing the fresh project map and starting from that whenever we call merge-profiles, unmerge-profiles or set-profiles. It also means your middleware functions shouldn’t have any non-idempotent side-effects since they could be called repeatedly.

If you need to include a profile in the project map, please add it as a plugin profile and ask your users to add it to the :base profile as outlined in the “Plugin” subsection of “Other Plugin Contents” in this document. This makes the “injection” more explicit and easier to debug. The only times one should use middleware to inject values into the project map is if the profiles has to be programmatically computed, or if you have to modify the project map in a way that is not possible with merge-profiles.

Note that middleware application will be memoized unless the :memoize-middleware? key is set to false.

Note: Leiningen supports loading middleware implicitly when the middleware is named plugin-name.plugin/middleware; however this mechanism is even more difficult to debug than regular middleware. It is strongly advised against using.

Maven Wagons

Pomegranate (the library used by Leiningen to resolve dependencies) supports registering “wagon” factories. Wagons are used to handle non-standard transport protocols for repositories, and are looked up based on the protocol of the repository url. If your plugin needs to register a wagon factory, it can do so by providing a leiningen/wagons.clj file containing a map of protocols to functions that return wagon instances for the protocol. For example, the following wagons.clj will register a wagon factory function for dav: urls:

{"dav" #(org.apache.maven.wagon.providers.webdav.WebDavWagon.)}

See S3 wagon private or lein-webdav for full examples of plugins using this technique.

VCS Methods

Leiningen ships with a vcs task which performs a handful of release-related version control tasks via multimethods. Out of the box it contains implementations for Git, but plugins can add support for more systems by including a leiningen.vcs.$SYSTEM namespace. All namespaces under the leiningen.vcs. prefix will be loaded when the vcs task is invoked. These namespaces should simply define methods for the defmultis in leiningen.vcs that invoke the specific version control system.

Requiring Plugins

To use a plugin in your project, just add a :plugins key to your project.clj with the same format as :dependencies. In addition to the options allowed by :dependencies, :plugins also allows you to disable auto-loading of hooks or middleware.

(defproject foo "0.1.0"
  :plugins [[lein-pprint "1.1.1"]
            [lein-foo "0.0.1" :hooks false]
            [lein-bar "0.0.1" :middleware false]])

Clojure Version

Leiningen 2.7.0 and on uses Clojure 1.8.0. If you need to use a different version of Clojure from within a Leiningen plugin, you can use eval-in-project with a dummy project argument:

(eval-in-project {:dependencies '[[org.clojure/clojure "1.4.0"]]}
                 '(println "hello from" *clojure-version*))

Projects vs Standalone Execution

Some Leiningen tasks can be executed from any directory (e.g. lein repl). Some only make sense in the context of a project.

To check whether Leiningen is running in the context of a project (that is, if a project.clj is present in the current directory), check for the :root key in the project map:

(if (:root project)
  (comment "Running in a project directory")
  (comment "Running standalone"))

If your plugin may run outside the context of the project entirely, you should still leave room in the arguments list for a project map; just expect that it will be nil if there’s no project present. Use ^:no-project-needed metadata to indicate this is acceptable.

In Leiningen 1.x, having a task function return a numeric value was a way to signal the process’s exit code. In Leiningen 2.x, tasks should call the leiningen.core.main/abort function when a fatal error is encountered. If the leiningen.core.main/*exit-process?* var is bound to true, then this will trigger an exit, but in some contexts (like with-profiles) it will simply trigger an exception and go on to the next task.

Overriding Built-in Tasks

Normally if you create a plugin containing (say) a leiningen.compile namespace, it won’t be used when lein compile is run; the built-in task will override it. If you’d like to shadow a built-in task, you can either create an alias or put it in the leiningen.plugin.compile namespace.

Project-specific Tasks

Occasionally, the need arises for a task to be included in a project’s codebase. However, this is much less common than people think. If you simply have some code that needs to be invoked from the command-line it’s much simpler to have your code run in a -main function inside your project and invoke it with an alias like lein garble:

:aliases {"garble" ["run" "-m" "myproject.garble" "supergarble"]}

Note that aliases vectors result in partially applied task functions, so with the above config, lein garble seventeen would be equivalent to lein run -m myproject.garble supergarble seventeen (or (myproject.garble/-main "supergarble" "seventeen") from the repl). The arguments in the alias are concatenated to the arguments provided when it’s invoked.

You only need to write a Leiningen task if you need to operate outside the context of your project, for instance if you need to adjust the project map before calling eval-in-project or some other task where you need direct access to Leiningen internals. You can even read values from the project map with an alias:

:aliases {"garble" ["run" "-m" "myproject.garble" :project/version]}

This will splice the value of the project map’s :version field into the argument list so that the -main function running inside the project code gets access to it.

The vast majority of these cases are already covered by existing plugins, but if you have a case that doesn’t exist and for some reason can’t spin it off into its own separate plugin, you can enable this behavior by placing the foo.clj file defining the new task in tasks/leiningen/ and add tasks to your .lein-classpath:

$ ls
README.md project.clj src tasks test
$ ls -R tasks

$ echo -ne ":tasks" | cat >> .lein-classpath
$ lein foo
Hello, Foo!

Note that in most cases it’s better to spin off tasks into their own plugin projects; using .lein-classpath is mainly appropriate for experimentation or cases when there isn’t enough time to create a proper plugin.

Have Fun

Please add your plugin to the list on the wiki once it’s ready.

Hopefully the plugin mechanism is simple and flexible enough to let you bend Leiningen to your will.