This article is part of the “Meta Advent 2019” series. I’ve committed to writing a new blog post here every day until Christmas.

A while ago I wrote an article explaining how the jack-in process in CIDER works, but I omitted one subject - what happens if your project has multiple project marker files (e.g. project.clj and deps.edn). This wasn’t a big issue a few years back, when almost everyone was using Leiningen, but now it’s pretty common to have some project with both Leiningen and tools.deps project marker files in it, or maybe tools.deps and shadow-cljs. So, how does CIDER handle such ambiguity and decide which tool to use when starting a REPL?

Well, by default it will simply prompt you to select one of the build tools it detected. Pretty simple, but also a bit annoying if you keep selecting the same tool all of the time. Fortunately, CIDER has an option to customize this though the cider-preferred-build-tool configuration variable.1 If you’re fond of Leiningen you can instruct CIDER to always prefer it like this:

(setq cider-preferred-build-tool 'lein)

In practice it will be common to have the preferred build tool depending on the project you’re working on, so an optimal approach to leverage this variable would be through project-specific configuration:

((clojure-mode
  (cider-preferred-build-tool . lein)))

In the end of the day you might combine the two approaches by putting in your global configuration (e.g. init.el) your favourite tool and overriding this default choice on a per-project basis via .dir-locals.el. That’s what I normally do, although frankly speaking, I restart my REPLs so rarely that I don’t have any issues being prompted about the build tool to use every time I do so.

That’s all I have for you today. See you again tomorrow!

Articles in the Series

  1. CIDER uses the “build tool” terminology for historical reasons, as tools.deps didn’t exist back then. ↩