The next release of Leiningen (likely 2.9.9) will include support for starting an nREPL server listening to an Unix domain socket.1 As a reminder - Unix domain sockets were one of the highlights of nREPL 0.9, which was released at the end of 2021.

The support for Unix sockets in Leiningen is two-fold. First, you’ll be able to do the following:

$ lein repl :headless :socket path/to/socket

This will start a headless nREPL, bound to the socket file you’ve specified. Alternatively, you’ll be able to specify a :socket option in your project.clj under :repl-options:

;; Options to change the way the REPL behaves.
:repl-options {  ;; Customize the socket the repl task listens on and
                 ;; attaches to.  Specify either a filesystem :socket
                 ;; (where the parent directory should be used to
                 ;; control access since POSIX doesn't require
                 ;; respecting the socket permissions):
                 :socket "/path/to/the/socket"
                 ;; or a network :host and/or :port
                 ;;   :host ""
                 ;;   :port 4001

That’s useful if you always want to use the same socket file.

The work done in Leiningen essentially wraps years of work, mostly carried out by Rob Browning, to perfect the support for Unix sockets in nREPL and CIDER. It was a long journey, but we made it! Now it’s time to sit back, relax and enjoy the fruits of this great work!2

  1. See

  2. Oh, well - we still need to teach CIDER how to extract the socket file automatically from Leiningen’s output, but that’s relatively simple.