Naming is hard! It truly is! I’ve spent a lot of time ruminating on the title for that particular post, before coming up with “nREPL Redux”. I could have just as easily gone with something like “nREPL Reborn”, “nREPL Returns”, “nREPL Forever”, “nREPL Strikes Back” or “The Revenge of nREPL”.1 Anyways, the name doesn’t matter that much - what matters is the story I’m about to tell you. Namely the story of the rise, fall and re-birth of the most important tool in the Clojure ecosystem today.

If you’re into Clojure development you probably know that I’m the author of CIDER - a (somewhat) popular Clojure programming environment. Most people think of CIDER as an Emacs project, but it’s much bigger than this - there are so many moving pieces that make the whole thing work, that I should probably write separately on this subject (and I totally plan to do so). One piece is special, though. It’s the very heart of CIDER and it’s the nREPL server which powers all the magic that happens behind the scenes - code evaluation, code completion, debugging and all that jazz.

nREPL is, very simply put, a network REPL server, which aims to provide a common ground for Clojure tool authors who need access to a REPL. This post is mostly about nREPL, but it’s also about some bigger topics like building inclusive communities, avoiding past mistakes and driving progress forward. So, here we go…

A Long Time Ago in a REPL Far Away….

In the early days of Clojure people would often praise the ingenious design of the language and the brilliance of Rich Hickey, but would shy away from the topic of development tools for Clojure, as there weren’t that many to go around.

A SLIME version tweaked for Clojure was the only real game in town early on, and Emacs quickly became the most popular editor for Clojure hackers. Later on, Eclipse and NetBeans plugins for Clojure emerged as well, and everyone rolled their own implementation of a REPL server that powered the interactions users had with Clojure.

I spoke about this at length at Clojure/conj 2014 and there are some really awesome nREPL design documents, like this one, that tell even more of the story.

nREPL was created to tackle the problem of the missing common infrastructure for Clojure development tools and it took the Clojure world by a storm. In the span of a couple of years (2010-2012) pretty much all the Clojure development tooling re-aligned itself around nREPL and the project achieved its ambitious original objective. I guess almost all of you have seen this:

$ lein repl

nREPL server started on port 54588 on host - nrepl://
REPL-y 0.3.7, nREPL 0.2.12
Clojure 1.8.0
Java HotSpot(TM) 64-Bit Server VM 10.0.1+10
    Docs: (doc function-name-here)
          (find-doc "part-of-name-here")
  Source: (source function-name-here)
 Javadoc: (javadoc java-object-or-class-here)
    Exit: Control+D or (exit) or (quit)
 Results: Stored in vars *1, *2, *3, an exception in *e


Most build tools today start nREPL servers and every editor/IDE out there provides some form of nREPL integration. CIDER itself was built with the explicit objective to replace SLIME with an nREPL-powered tool. Some of you might even remember that the first few versions of CIDER were actually named nREPL.el.2

Clojure Contrib Strikes Back

nREPL started as a personal project of Chas Emerick @ in 2010. After its initial success, it was invited to be one of the initial three projects in the “new” Clojure Contrib family of projects later that year, and thus moved to

This was supposed to be a step towards nREPL becoming part of Clojure at some point, but alas - things took a pretty different turn. The project was doing great while Chas was committed to it, but his life and work priorities shifted and the development slowed down significantly. I guess you might be wondering why would something like this happen? Didn’t I just say that nREPL was a crucial piece of the Clojure ecosystem? After all it’s open-source software and there should be people willing to contribute, right?

In theory - yes, in practice - not so much. I might be completely off when I say this, but I attribute the lack of contributors mostly to the heavy-handed contribution process for Clojure Contrib projects. For those of you who are not familiar with Contrib, here are the highlights of this process:

  • Code is hosted on GitHub.
  • You need sign a contributor agreement (CA) to contribute.
  • You have to use JIRA for issues and patches. Collaboration via GitHub Tickets/PRs is a no go.
  • You have to use Hudson (!!!) for CI.
  • You have to use Maven (all contrib projects share a common Maven template).

You can imagine that most people (me included) are not exactly thrilled to jump though all those hoops. The numbers speak for themselves - most Clojure Contrib projects have between 1 and 0 maintainer. Sometimes the original authors of key projects would lose interest in the project, but no one would take over. My guess as to why this is happening is the nature of the contribution process.

I also find it somewhat amusing that no project in Contrib has ever been promoted to Clojure itself, so this really begs the question of what exactly is the goal for the entire endeavour, but I’ll leave this subject for another article.

A New Hope

In the mean time I was trying to build a great Clojure IDE on top of nREPL and I was getting really frustrated with the nREPL situation, as many of the features I wanted to build or problems I wanted to address required changes to nREPL. I was really willing to contribute, but I definitely value my limited time way too much to be wasting it on a process with which I completely disagree.

The people who know me are aware of my immense passion for free software projects and that I always want to build healthy communities around my projects, nurture contributors, etc. Clojure Contrib goes against everything I hold near and dear! I could have probably gotten commit access to the nREPL project in Clojure Contrib, but this wouldn’t have been a solution to the real problem (an environment that discourages contributions), so I opted not to go in this direction. Instead I decided to go all in on this and to propose something novel and somewhat unorthodox…

I started advocating strongly for migrating nREPL out of Clojure Contrib3 and rebooting the project’s development in a truly free and welcoming manner. Eventually, Chas agreed that’s the reasonable course of action and announced his intention to migrate nREPL out of Clojure Contrib in the summer of 2017 (, with the objective of reconstituting an environment where the people most interested in and impacted by nREPL could participate in its development as easily as possible.

You have no idea how excited I was about all of this! However, things are never as easy as they sound. Little did I know back then that I’ll be the one who’ll actually have to carry out the migration.

Rogue One

Chas did some of the groundwork for migrating nREPL out (he re-licensed the project, re-created the Git repo, expunging all the irrelevant commits, etc), but then he ran out of time for this and I offered my assistance with the outstanding tasks. I wanted to show that I’m not just one of those people who complain and don’t mean what they say. nREPL was truly free at this point and this meant that I had no reservations to jump in and help out. Still, I wish I understood better what I was signing up for back then - it was way more work then I could possibly imagine…

The first steps were pretty easy and clear:

  • We needed to “rebrand” the project and change its deployment artifact and namespaces, as technically this was a new project (even though in spirit it was a continuation of the old one).
  • We needed to update all popular extensions (middleware and transports mostly) to support the new nREPL.
  • We needed to fix some bugs and implement some long overdue features.
  • We needed to clean up some legacy code.

At the time, however, I didn’t realize that there would also be a lot of effort to update nREPL in various downstream tools

  • in particular Leiningen and Boot. Unfortunately releases there don’t happen very often and while the latest stable Boot switched to the new nREPL already, Leiningen still has to cut a release targeting it. This meant I had to do a lot of extra work to support both tools.nrepl and the new nREPL in many projects.

I also didn’t anticipate that a lot of the key nREPL middleware were also somewhat unmaintained and I had to personally go and update all of them (Piggieback, refactor-nrepl, sayid, etc). Same with the popular Drawbridge HTTP transport. Eventually this lead to the creation of a new nREPL GitHub org, that houses the “core” nREPL projects and to me getting really familiar with nREPL and its ecosystem. So, familiar that at some point I even decided to create a proper manual for nREPL, which is one of the most time-consuming tasks I’ve been working on lately.

All of this resulted in an extremely busy and frustrating summer for me and a lot of work I didn’t see coming, but I’m really glad I managed to pull through and now we’re finally starting to reap the benefits of it all. Right now every meaningful piece of the Clojure development ecosystem is compatible with the new nREPL and I hope that in a year or so we’ll get rid of all the compatibility code and clean up shop a bit.

I want to extend a special “Thanks!” to my friends in the London Clojurian community and Clojurists Together who kept me going, even when I had lost hope that what I set out to do was doable. I couldn’t have made it without you!

Now I feel as burned out as shit, but I’m also extremely happy that I got us where I wanted us to be!

Interlude - The Socket REPL

I believe some of you might be wondering if all this hard work was really worth it. After all Clojure now has the Socket REPL and if that’s part of Clojure probably we’re supposed to use it, right?

There are also some interesting projects like unrepl and prepl that build on top of it to provide a similar functionality to the one offered by nREPL.

I saw a couple of days ago an amazing tweet by Joe Armstrong (of Erlang fame), that perfectly reflects how I feel about all of this:

The groundhog day development method

1) Invent a new simple thing

2) Add features

3) Add more features

4) Add more and more and more and more features

N-2) It’s so complicated nobody can use it

N-1) Throw everything away

N) goto 1)

Is there a way out?

Maybe :-)

– Joe Armstrong

Technical merits and personal preferences aside, people should understand that the Clojure community has made an epic investment in building tools of top of nREPL and that going back to 1) will set us back a lot. I know I’m never putting in the work to add support for the Socket REPL to CIDER myself. I believe that nREPL is exactly what I need and with a little bit of work to enhance it, we can get a lot of more of value for a fraction of the investment. And we can do this today.

I’ve put in a lot of work to make sure the underpinnings of CIDER like orchard, cljs-tooling, etc are not nREPL specific and other tool authors can leverage those, so no one has to re-invent the wheel again, but that being said, I really believe that we already have the flexible and solid foundation that we need and we should just work towards making it better.

There’s also the fact that unlike the Socket REPL and prepl, nREPL is free from the shackles of the Clojure (Contrib) contribution process. Don’t get me wrong - the fact that I’m (often) critical of Contrib doesn’t mean I’m disrespectful to Rich’s and Cognitect’s decisions and I think they are doing something inherently wrong. I just think that what works for Clojure and projects that might (really) end up being part of Clojure (e.g. tools.reader, core.async), doesn’t really make sense for something like a REPL server. I think that we really should try to have as little functionality as possible tied directly to Clojure, so we can be more nimble and flexible when it comes to fixing and improving things. Probably if the Socket REPL was at least a Contrib project I’d have more faith in it, but I definitely don’t want to wait one year for changes that can be done today, and perhaps some of you share my perspective on this.

nREPL is that solution. Free of Clojure’s process, free of Clojure’s release cycle, open for business. This post has several goals and the most important one of them is to invite all of you to help get nREPL to the next level.

On top of this - nREPL is also mature, stable, battle tested and with an incredibly flexible design. I think that 90% of the people who complain about nREPL’s limitations simply don’t understand how easy it is to do with it anything you can possible wish for. The transports abstraction, in particular, is mind-blowing.

The Small Victories

In the case you’re wondering how much the new project has achieved so far, I’ll let the changelog speak for that. In a nutshell:

  • We’ve fixed plenty of annoying bugs.
  • We now have awesome support for using nREPL together with clj and tools.deps.
  • We have awesome nREPL documentation.
  • We’ve added support for hot-loading dependencies in nREPL.4
  • We’re close to adding authorization support to nREPL. (PR)
  • We’re close to shipping host-wide and project-specific config support for nREPL. (PR)
  • We’re about to ship built-in support for pretty-printing.

And we did all of this without breaking any backwards compatibility (apart from the unfortunately namespace changes that were unavoidable). We’re committed to continue to develop the project is a stable and responsible fashion.

I’ve got a small hint for you - nREPL 0.5 is going to be big!

And yeah - I used “we” instead of “I” intentionally. But more about this in a moment…

The Big Victories (The Return of the Jedi)

The biggest validation of all the work that went into re-booting nREPL is the amazing involvement of the community. Most of the nicest improvements were not contributed by me and that re-affirms my belief that if you create the right environment, people would rally around and drive progress and innovation forward.

I hope that down the road a new steward for the project will emerge and I’ll be able to take a step back into the shadows and switch back my focus to hacking on CIDER. I don’t see myself as the long-term maintainer of nREPL - I see myself simply as an agent of change, whose main objective is to set the project up for success.

We’re just getting started and I’m sure the best is yet to come!

Help Wanted

All of the projects in the nREPL org could benefit from more maintainers. It’s especially important to find stewards for key projects like Piggieback (ClojureScript support for nREPL) and Drawbridge (HTTP support for nREPL).

We’re also looking for someone willing to port nrepl.bencode to ClojureScript, so it’s easier to write ClojureScript clients for nREPL. That’s pretty much the only reason why we’ve extracted the bencode functionality out of nREPLs core.

nREPL’s manual needs proofreading, editing and more content (especially when it comes to documenting how to extend nREPL with new middleware and transports).

The project still needs a logo and we really want to add a couple of extra transports - e.g. EDN and Transit.

There’s also the big dream to port nREPL to ClojureScript as well, so it can be used in self-hosted environments. How awesome would that be, right?

There’s plenty of work around and no contribution is too small! Join us in nREPLness!


Telling stories like that one is the main reason I decided to revive my writing efforts. I think they truly are worth sharing.

That’s from me, folks! Keep hacking!

  1. Probably by now you’ve realized I’m a huge Batman and Star Wars fan. 

  2. Naming is hard! 

  3. To my knowledge this had never happened in the past. 

  4. That’s big! Take a look here for details.