Hot on the heals of yesterday’s article I’ve got a new one for you! This one is going to be short and sweet - just a few notes about the release of CIDER 0.20 (Oslo).

I guess some of you are surprised by the news - after all wasn’t CIDER 0.19 (Raleigh) released only a couple of weeks ago? Usually CIDER releases are (many) months apart. Well, while that’s normally the case, I’ve always wanted to do more focused and frequent releases and it seems that I managed to succeed this time around.

CIDER 0.20 includes very few changes overall.1 I’ll cover them briefly here.

Cleaning up Shop

CIDER (or cider-nrepl to be precise) dropped support for nREPL 0.2 (a.k.a. tools.nrepl). Now you’ll need nREPL 0.4+ (ideally 0.5.3+), but that’s not a big deal as at this point both Boot and Leiningen ship the modern nREPL. Just make sure you’re on Boot 2.8.2+ or Leiningen 2.8.3+.

I’ll be gradually removing support for nREPL 0.2 from all nREPL extensions I’m maintaining in the months to come. I’d advise other people maintaining nREPL extensions to do the same.

Native nREPL Pretty-printing

CIDER used to rely on a custom middleware for result pretty-printing for many years now, but nREPL recently added native support for pretty-printing values, and this rendered our custom approach redundant. The biggest change in CIDER 0.20 is that it switches to nREPL’s built-in pretty-printing API.

Some other related changes:

  • Support for zprint has been added.
  • We’re passing to the print engines (e.g. zprint) their native options, instead of wrapping their invocations in dynamic bindings (e.g. *print-length*, *print-level*, etc).
  • Pretty-printing is enabled in REPL buffers by default.
  • You can pretty-print results in scratch buffers using C-u C-j.

There’s now some work underway in nREPL to support streamed printing of values. That’s going to be a really great improvement of the current situation as it would mean faster feedback for the clients and no possibility for “invisible” printing of infinite/huge sequences on the server-side.2

CIDER now has a new section of its manual dedicated to pretty-printing. You should definitely check it out!

Note that the new pretty-printing requires for you to be on nREPL 0.5+. If you’re not, you’re simply going to get results that are not pretty-printed.


I’ve spent a bit of time reorganizing CIDER’s manual and breaking down some ridiculously big sections (e.g. Using the REPL, Configuration, etc) into smaller chunks. I think you’ll find the new layout easier to navigate, but there’s still a lot of room for improvement.

A handful of small changes and bug improvements also made it in this release, but there’s nothing worth highlighting there. Just the usual quality of life improvements you’re bound to expect from each new release.

On a related note - there’s also a new clojure-mode release, that you should certainly check out!


I don’t have a specific plan for the next release - neither in terms of features nor in terms of a timeline.

I’ll be taking some time off from working on open-source now that the new CIDER is out, and I hope that in the mean time the work on the next nREPL release will be done and I’ll just have to incorporate all that awesomeness into CIDER when I’m back.

As some of you probably know, Christophe Grand has joined nREPL’s team and is working on bringing some amazing features from unrepl to nREPL:

(Extremely) Exciting times ahead! I cannot thank Christophe enough for the amazing work he has been doing on nREPL recently! People like him are the heart and soul of the Clojure community and the reason why I love being part of it!


The year had an explosive start for me - so many project release, so much open-source work done! I was really eager to start “clean” and finish many of the initiatives that have been dragging throughout the past year. It was pretty painful and exhausting, but I got to a point where I did everything I felt I needed to do (for the time being) and now I’m looking forward to some respite and re-energizing myself.

While, I’m gone the development won’t stop, of course. In my absence the other CIDER and nREPL Core Team members will be reviewing PRs and cutting releases. I hope you’ll surprise me with many great developments next time I open my GitHub!

Enjoy CIDER Oslo (ir)responsibly! Keep hacking!


  2. Those are hard to spot and result in out of memory errors in nREPL.