Leiningen is great for pure Clojure projects. So why would one be interested in other Clojure build tools? I've been working with full-stack Clojure project for some months now. Figwheel provides nice workflow with live reloading for ClojureScript projects but from the start of the project I felt Lein leaves some things to be desired. Boot is a new build tool for Clojure. And I'll show how it improves on some areas in comparision to Leiningen.

1. Build tooling with Boot
Juho Teperi / Metosin
ClojuTRE
25.11.2014

2. Contents
• Example project
• Challenges with Leiningen
• Solutions using Boot
• Summary

3. Example project
• “(Typical) full-stack Clojure project”
• src/clj, backend code
• src/cljs
• src/cljx, shared code
• src/less (or sccs, garden etc.)

4. Leiningen
• Configuration over code
• Great with pure Clojure
projects
• Plugins provide additional
tasks

5. Example project using Lein
• Uses multiple lein plugins
• lein cljx auto
• lein cljsbuild auto (or figwheel)
• lein less auto
• lein repl
• + unit tests
• Several of these plugins watch for file changes
• There are ways to combine all into one
command

6. Lein, problem #1
• Each lein command starts a JVM
• Most commands fork another for the task
• 7 JVMs
• much GB
• Slow start up

7. Lein, problem #2
• Plugins are not composable
• Descriptive configuration doesn’t help
• Each lein plugin implements the file watching
themselves
• Each implementation works differently…

8. Lein, problem #3
• Target directory is in effect a temporary
directory
• Lein uberjar
• Uses different cljs compiler options
• Probably uses same target path
• First thing to try if you have problems: lein
clean

9. :source-paths ["src/clj" "target/generated/clj"]
:test-paths ["test/clj"]
:cljsbuild {:builds {:app {:source-paths ["src/cljs" "target/generated/cljs"]
:compiler {:output-to "resources/public/js/bootcamp.js"
:output-dir "resources/public/js/out"
:source-map "resources/public/js/out.js.map"
:preamble ["react/react.min.js"]
:externs ["react/externs/react.js"]
:optimizations :none
:pretty-print true}}}}
:profiles {:dev {:plugins [[lein-figwheel "0.1.5-SNAPSHOT"]
[lein-cljsbuild "1.0.3"]
[com.keminglabs/cljx "0.4.0" :exclusions [org.clojure/clojure]]]
:figwheel {:http-server-root "public"
:port 3449
:css-dirs ["resources/public/css"]}
:cljx {:builds [{:rules :clj
:source-paths ["src/cljx"]
:output-path "target/generated/clj"}
{:rules :cljs
:source-paths ["src/cljx"]
:output-path "target/generated/cljs"}]}
:cljsbuild {:builds {:app {:source-paths ["dev-src/cljs"]}}}}
+ This is missing e.g. uberjar
settings for optimized cljs build

10. Boot (2)
• Alternative build tool
• Boot 1 is the old version
• Boot 2 is the new one. It’s a significantly
different as the old one.
• Alpha
• API changes are imminent etc.
• Active development

11. Boot, solution #1
• “Lein launches multiple JVMs and uses lots of memory”
• All tasks run in the same JVM
• Separate classloaders to prevent deps leaking
• Btw. this is the same technique which is used by the app servers
• Disclaimer: I have not tested the memory consumption.

12. Boot, solution #2
• “Lein tasks are not composable, each plugin implements
e.g. file watching themselves”
• Tasks are functions
• New tasks are composed from existing tasks
using regular methods: comp
• File watching is a task
• Which can be composed together with other tasks

13. Boot, solution #3
• “Dev and production tasks interfere because they use
same directories”
• Tasks create temporary directories as needed
• Tasks use Boot API to find sources and publish their results
• Cljx uses API to create temp dir, writes results to the dir...
• Cljs asks for source files using the API -> sources include those
written by cljx task
• Processes don’t interfere with each other
• running boot package to create production jar shouldn’t leave
around files which break dev build
• No need for clean-task

14. (set-env!
:src-paths #{"src/clj" "src/cljs" "src/cljx"}
:rsc-paths #{"resources"}
:dependencies '[[adzerk/boot-cljs "0.0-2371-27"]
[adzerk/boot-cljs-repl "0.1.6"]
[adzerk/boot-reload "0.1.6"]
[deraen/boot-cljx "0.1.0-SNAPSHOT"]
… project deps … ])
(task-options!
cljs [:output-to "public/main.js"
:source-map true
:unified true])
(deftask dev
"Start the dev env..."
[]
(comp
Global options
● Watch: all following tasks run whenever there is file
change.
● cljs-repl, start-app are implemented so that they
only run once.
● (start-app is specific to the project)
● cljs, cljx no-op if no file changes for them
● Reload: Like figwheel. Sends websocket
(watch)
(cljs-repl)
(cljx)
(start-app)
(cljs :optimizations :none)
(reload :on-jsload 'saapas.core/main)))
notifications to browser.
(deftask dev-repl
"Connect to the repl started by the dev task."
[]
(repl :client true))

15. Challenges with Boot
• IDE Support
• Editors using nrepl work (fireplace, cider)
• Keep project.clj around for Cursive (and
maybe others)
• Missing tasks (less, unit testing…)
• boot-cljx was easy to create, so should be
the others

16. Summary
Lein Boot
Task envs Forks new JVMs Multiple classloaders in
one JVM
Configuration Configuration over
code
Composable functions
Files Static classpath
(target, resources)
Tasks create temp dirs
Equivalent JS tool Grunt Gulp, Broccoli

17. Questions?

18. Links
• http://boot-clj.com/
• https://github.com/Deraen/saapas
• #hoplon @ Freenode