The nREPL server from babashka as a library, so it can be used from other sci-based CLIs.

Sci is available here: https://github.com/borkdude/sci

To start a server, call babashka.nrepl.server/start-server!. To stop a server call babashka.nrepl.server/stop-server!

Before you start a server you will need to create a sci context:

(require '[sci.core :as sci])
(require '[sci.addons :as addons])

(def opts (-> {:namespaces {'foo.bar {'x 1}}} addons/future))
(def sci-ctx (sci/init opts))

A sci context is derived from options as documented in sci.core/eval-string and contains the runtime state of a sci session. This context is re-used over successive REPL invocations.

To start an nREPL server in your project, call babashka.nrepl.server/start-server!. The call takes two arguments, your initial sci context and some options including the IP address to bind to, the port number and optional debug and quiet flags. E.g.:

(babashka.nrepl.server/start-server! sci-ctx {:host "127.0.0.1" :port 23456})
;; Started nREPL server at 127.0.0.1:23456

If :debug is set to true, the nREPL server will print to stdout all the messages it is receiving over the nREPL channel.

If :debug-send is set to true, the server will also print the messages it is sending.

if :quiet is set to true, the nREPL server will not print out the message "starting nREPL server at...". If not specified then :quiet defaults to false, and the message will be printed.

If :port is not specified, it defaults to 1667.

If :host is not specified, it defaults to 0.0.0.0 (bind to every interface).

Options can contain a :describe map which will be merged in with the response for the :describe op.

If no options hashmap is specified at all, all the defaults will be used. Thus the following is a valid way to launch an nREPL server.

(babashka.nrepl.server/start-server! sci-ctx)
;; Started nREPL server at 0.0.0.0:1667

Pass the result you received from start-server! to stop-server! to shut down the server.

(->
  (babashka.nrepl.server/start-server! sci-ctx)
  (babashka.nrepl.server/stop-server!))
Started nREPL server at 0.0.0.0:1667
nil

Use babashka.nrepl.server/parse-opt to parse strings like:

(babashka.nrepl.server/parse-opt "localhost:1667")
;;=> {:host "localhost", :port 1667}
(babashka.nrepl.server/parse-opt "1667")
;;=> {:host nil, :port 1667}

You can pass the return value of parse-opt to start-server!:

(babashka.nrepl.server/start-server!
    sci-ctx
    (babashka.nrepl.server/parse-opt "localhost:23456"))

Often you will want to launch the server and then block execution until the server is shutdown (at which point the code will continue executing), or ctrl-C is pressed (at which point the process will exit). This can be achieved as follows:

(babashka.nrepl.server/start-server! sci-ctx {:host "127.0.0.1"
                                              :port 1667})
@(promise)

Connecting to the nREPL from CIDER gives:

;; nREPL:
clojure.lang.ExceptionInfo: Could not resolve symbol: clojure.main/repl-requires [at line 1, column 42]

This is because some nREPL clients use clojure.main/repl-requires to find a list of automatic requires to run at the beginning of the nREPL. Simply supply a value in you sci bound namespace for this value:

(-> sci-ctx
    (assoc-in [:namespaces 'clojure.main 'repl-requires]
        '[[clojure.repl :refer [dir doc]]])
    sci.addons/future
    sci.core/init
    babashka.nrepl.server/start-server!)

The main body of work was done by Michiel Borkent (@borkdude). Addition rework and some added functionality was done by Crispin Wellington (@retrogradeorbit).

The project code is Copyright © 2019-2020 Michiel Borkent

It is distributed under the Eclipse Public License 1.0 (http://opensource.org/licenses/eclipse-1.0.php)