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Launchpad is a Clojure dev process launcher.

It starts from these observations:

  • Clojure development is done interactively
  • This requires a nREPL connection between a Clojure process and an editor
  • How Clojure/nREPL gets started varies by
    • editor (which middleware to include?)
    • project (how to start the system, cljs config)
    • individual (preferences in tooling, local roots)
  • Projects are increasingly multi-module, either monorepo, or spread across repos
  • We mainly rely on our editors to launch Clojure/nREPL because it is tedious
  • Other tools could benefit from participating in the startup sequence (e.g. lambdaisland/classpath)
  • Automating startup is done in an editor-specific way (.dir-locals.el, calva.replConnectSequences)
  • And requires copying boilerplate around (user.clj)

And these preferences:

  • We want project setup to be self-contained, so starting a process "just works"
  • This should work for everyone on the team, no matter what editor they use
  • We prefer running the process in a terminal for cleaner separation and control
  • When working on multiple related projects we prefer a single JVM over multiple

How it works

Launchpad is a babashka-compatible library, through a number of steps it builds up a Clojure command line invocation, including JVM options, extra dependencies, and forms to evaluate, including starting nREPL with selected middleware, watching deps.edn files for changes, starting ClojureScript build watchers, and hooking up your editor (if supported, otherwise a manual connect to nREPL is needed).

It takes information from deps.edn (checked in) and deps.local.edn (not checked in) and arguments passed in on the command line to determine which modules to include, which middleware to add to nREPL, which shadow-cljs builds to start, etc.

Launchpad integrates with lambdaisland.classpath to provide hot-reloading of deps.edn and deps.local.edn, so you can add dependencies to your project, switch a dependency to a newer version, or from a Maven-provided jar to a local checkout, all without restarting your process and REPL. You can even activate additional aliases without restarting. How cool is that?

Project setup

See template for an example setup, you need a few different pieces.

  • bb.edn
{:deps {com.lambdaisland/launchpad { ... }}}
  • bin/launchpad

This is the default convention for launchpad-based projects, it provides a recognizable, predictable entry point that people will be looking for in your project, so use it. This is a simple babashka script invoking launchpad, but there's lots of room to customize this. Want to check the Java version, launch docker-compose, ensure environment variables are set up? You do that here.

#!/usr/bin/env bb

(require '[lambdaisland.launchpad :as launchpad])

(launchpad/main {})

;; (launchpad/main {:steps (into [(partial launchpad/ensure-java-version 17)]
;;                               launchpad/default-steps)})
  • deps.edn

You need a top-level deps.edn, where you reference your sub-projects with aliases and :local/root.

 {... good place for dev tooling like portal, scope-capture ...}

 ;; Monorepo setup with two subprojects, if you have a multi-repo setup then use
 ;; paths like `"../proj1"`
  {:deps {com.example/proj1 {:local/root "proj1"}}}

  {:deps {com.example/proj2 {:local/root "proj2"}}}}}

proj1 and proj2 are then folders with their own deps.edn, src, etc.

  • .gitignore

Make sure to .gitignore the deps.local.edn file, this is where you can do individual configuration.

echo deps.local.edn >> .gitignore
  • deps.local.edn

This follows the structure like deps.edn, and gets passed to tools.deps as an extra source, but there are a few special keys here that you can use to configure launchpad.

{;; regular deps.edn stuff will work in here
 :deps {}
 :aliases {}

 ;; but some extra keys are supported to influence launchpad itself
 :launchpad/aliases [:proj1] ; additional aliases, will be added to whatever
                             ; aliases you specify on the command line
 :launchpad/main-opts ["--emacs"] ; additional CLI flags, so you can encode your
                                  ; own preferences
 :launchpad/shadow-build-ids [] ; which shadow builds to start, although it may
                                ; be preferable to configure this as part of
                                ; specific aliases in your main deps.edn
 ;; which shadow builds to automatically connect to if `--emacs` flag is provided
 :launchpad/shadow-connect-ids [] 

You don't have to stop there, you could add a dev/user.clj (add "dev" to your :paths in that case), add other useful scripts or repl sessions, maybe you even want to put cross-project integration tests in this repo, but the above is the main stuff you need.

Using Launchpad

When invoking bin/launchpad you pass it any aliases you want to start, plus a number of optional flags. These currently allow injecting the CIDER and/or refactor-nrepl middleware, which will suffice to use launchpad with Emacs/CIDER, VS Code/Calva, and probably others.

Please do file issues on Github for your favorite editor environment, we want to eventually support every Clojure editor out there with a non-negligible user base.

Emacs is currently best supported, since we are able to query Emacs to find which versions of cider-nrepl and refactor-nrepl we should inject, and are also able to instruct Emacs to connect to the REPL, so the whole process is smooth and automated. This level of integration will not be possible with every editor, but we can look into what options we have.

➜ bin/launchpad --help
bin/launchpad <options> [proj1|proj2]+

  -h, --help
  -v, --verbose         Print debug information
      --cider-nrepl     Include the CIDER nREPL middleware
      --refactor-nrepl  Include the refactor-nrepl middleware
      --cider-connect   Automatically connect CIDER
      --emacs           Shorthand for --cider-nrepl --refactor-nrepl --cider-connect

For example

➜ bin/launchpad backend frontend --emacs
[INFO] clojure -J-XX:-OmitStackTraceInFastThrow -J-Dlambdaisland.launchpad.aliases=backend,frontend -A:backend:frontend -Sdeps '{:deps {cider/cider-nrepl #:mvn{:version "0.28.5"}, refactor-nrepl/refactor-nrepl #:mvn{:version "3.5.2"}, com.lambdaisland/classpath #:mvn{:version "0.4.44"}, com.lambdaisland/launchpad #:local{:root "/home/arne/github/lambdaisland/launchpad"}}}' -M -e '(do (require (quote (quote lambdaisland.launchpad.deps)) (! {:aliases [:backend :frontend], :include-local-roots? true, :basis-fn lambdaisland.launchpad.deps/basis, :watch-paths [( "deps.local.edn")], :launchpad/extra-deps (quote {cider/cider-nrepl #:mvn{:version "0.28.5"}, refactor-nrepl/refactor-nrepl #:mvn{:version "3.5.2"}, com.lambdaisland/classpath #:mvn{:version "0.4.44"}})}) (require (quote nrepl.cmdline)) (nrepl.cmdline/-main "--port" "41545" "--middleware" "[cider.nrepl/cider-middleware refactor-nrepl.middleware/wrap-refactor]"))'
nREPL server started on port 41545 on host localhost - nrepl://localhost:41545

At this point in Emacs you'll see this pop up:

[nREPL] Establishing direct connection to localhost:41545 ...
[nREPL] Direct connection to localhost:41545 established

For other editors connect to the given port manually.

Writing custom steps

Launchpad performs a number of steps, currently these are

  • read-deps-edn : Read in deps.edn, ~/.clojure/deps.edn, and deps.local.edn
  • handle-cli-args : Parse CLI arguments
  • get-nrepl-port : Figure out the nREPL port, either from a CLI argument or use a free port
  • get-nrepl-bind : Figure out which device to bind to
  • compute-middleware : Figure out the nREPL middleware to use (optionally CIDER, refactor-nrepl, and whatever else is configured)
  • compute-extra-deps : Add additional dependencies if needed, like nREPL, CIDER, refactor-nrepl
  • include-hot-reload-deps : Add lambdaisland.classpath and set it up for hot reloading deps.edn
  • include-launchpad-deps : Add launchpad itself as a dependency, since it contains runtime helpers for dotenv and shadow-cljs handling
  • watch-dotenv : Setup watch handlers to hot-reload .env and .env.local
  • start-shadow-build : Start the shadow-cljs build process, if necessary
  • maybe-go : Add (user/go) to the list of code to run on startup
  • disable-stack-trace-elision : Add -XX:-OmitStackTraceInFastThrow to the JVM flags
  • inject-aliases-as-property : Add -Dlambdaisland.launchpad.aliases as a JVM flag, so we know at runtime which aliases we started with
  • include-watcher : Start up the main watcher which will handle deps.edn/.env watching
  • run-nrepl-server : Add startup code to start nREPL
  • print-summary : Print an overview of aliases and extra dependencies that are being included
  • start-process : This is where we actually start Clojure
  • wait-for-nrepl : Wait for nREPL to be available
  • maybe-connect-emacs : Instruct Emacs to connect to nREPL

Each steps works on a context (ctx) map, mostly what you do is add stuff into that context, which gets used to construct the final clojure/JVM startup command. These are some of the keys in the context that you can read, set, or update:

  • :java-args : sequence of JVM arguments, without the leading -J
  • :requires : namespaces to load (sequence of symbols)
  • :eval-forms : code to evaluate (sequence of forms)
  • :options : the result of parsing command line flags (map)
  • :watch-handlers : map from file path to filesystem change handler function
  • :extra-deps : additional dependencies to load, map from artifact name (symbol), to deps.edn coordinates map
  • :env : environment variables to set (gets populated with .env/.env.local)
  • :middleware : nREPL middleware to include, sequence of fully qualified symbols
  • :shadow-cljs/build-ids : shadow-cljs build-ids to start
  • :shadow-cljs/connect-ids : shadow-cljs build-ids to connect a REPL to
  • :clojure-process : the Java Process object, once Clojure has started
  • :nrepl-port : nREPL TCP port
  • :nrepl-bind : nREPL device IP
  • :aliases : deps.edn aliases in use
  • :main-opts : CLI command line options (seq of string)
  • :deps-edn : merged deps.edn map
  • :paths : paths to add to the classpath

Most of the time you want to add extra steps either right before, or right after start-process. The vars before-steps and after-steps are useful for that.

(require '[lambdaisland.launchpad :as launchpad]
         '[babashka.process :refer [process]])

(defn npm-install [ctx]
  (process '[npm install] {:out :inherit
                           :err :inherit})

  (concat launchpad/before-steps

Lambda Island Open Source


launchpad is part of a growing collection of quality Clojure libraries created and maintained by the fine folks at Gaiwan.

Pay it forward by becoming a backer on our Open Collective, so that we may continue to enjoy a thriving Clojure ecosystem.

You can find an overview of our projects at lambdaisland/open-source.




Launchpad is not currently open to external contributions.


Copyright © 2022 Arne Brasseur and Contributors

Licensed under the term of the Mozilla Public License 2.0, see LICENSE.

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