Usage

Starting a server

There are many ways to start an nREPL server. Most often you’d start it using some build tool, but you can also embed the server in your application and start it from it. Here we’ll outline the most popular options available for you today.

Using Leiningen

Leiningen has built-in support for nREPL since version 2. Just do:

lein repl

And you’re all set. By default Lein will also connect to the running nREPL server using the popular command-line nREPL client REPL-y. If you don’t need the terminal REPL you can also start nREPL in headless mode:

lein repl :headless

Using Boot

Boot is a popular alternative to Leiningen, that also has build-in support for nREPL:

boot repl

Using Clojure CLI tools

If you’re into the clj command you can take advantage of nREPL’s built-in command-line interface (nrepl.cmdline).

Add this alias to ~/.clojure/deps.edn:

{
;; ...
:aliases {:nREPL
          {:extra-deps
            {nrepl/nrepl {:mvn/version "0.4.5"}}}}
}

Then you can simply run the nREPL server in headless mode like this:

clj -R:nREPL -m nrepl.cmdline

A good practice is add whatever nREPL middleware you want to use to the nREPL profile, so you can easily activate them when needed. Here’s how you can easily start a ClojureScript capable nREPL:

{
;; ...
:aliases {:nREPL
          {:extra-deps
           {nrepl/nrepl {:mvn/version "0.4.5"}
            cider/piggieback {:mvn/version "0.3.8"}}}}
}

clj -R:nREPL -m nrepl.cmdline --middleware "[cider.piggieback/wrap-cljs-repl]"

Here’s a listing of all the options available via nREPL’s command-line interface (this output was simply generated with --help):

-i/--interactive            Start nREPL and connect to it with the built-in client.
-c/--connect                Connect to a running nREPL with the built-in client.
-C/--color                  Use colors to differentiate values from output in the REPL. Must be combined with --interactive.
-b/--bind ADDR              Bind address, by default "::" (falling back to "localhost" if "::" isn't resolved by the underlying network stack).
-h/--host ADDR              Host address to connect to when using --connect. Defaults to "localhost".
-p/--port PORT              Start nREPL on PORT. Defaults to 0 (random port) if not specified.
--ack ACK-PORT              Acknowledge the port of this server to another nREPL server running on ACK-PORT.
-n/--handler HANDLER        The nREPL message handler to use for each incoming connection; defaults to the result of `(nrepl.server/default-handler)`.
-m/--middleware MIDDLEWARE  A sequence of vars, representing middleware you wish to mix in to the nREPL handler.
--help                      Show this help message.

Connecting to an nREPL server

Most of the time, you will connect to an nREPL server using an existing client/tool. Tools that support nREPL include:

CIDER (Clojure Interactive Development Environment that Rocks for Emacs)
Cursive (Clojure IDE/plugin for IntelliJ Idea)
Counterclockwise (Clojure IDE/plugin for Eclipse)
monroe (nREPL client for Emacs)
fireplace.vim (Clojure + nREPL support for vim)
Proto REPL (Proto REPL is a Clojure development environment and REPL for Atom)
Calva (Clojure & ClojureScript support for VS Code)
REPL-y (command-line client for nREPL)

Both Leiningen and Boot use internally REPL-y, as their command-line nREPL client.

If your preferred Clojure development environment supports nREPL, you’re done. Use it or connect to an existing nREPL endpoint, and you’re done.

Using the built-in client

nREPL ships with a very simple command-line client that you can use for some basic interactions with the server. The following command will start an nREPL server and connect with it using the built-in client.

clj -Sdeps '{:deps {nrepl {:mvn/version "0.4.5"}}}' -m nrepl.cmdline --interactive
nREPL server started on port 59403 on host 0:0:0:0:0:0:0:0 - nrepl://0:0:0:0:0:0:0:0:59403
nREPL 0.4.5
Clojure 1.9.0
Java HotSpot(TM) 64-Bit Server VM 10.0.1+10
user=> (+ 1 2)
3

If you want to connect to a server that’s already running you can do it like this:

clj -Sdeps '{:deps {nrepl {:mvn/version "0.4.5"}}}' -m nrepl.cmdline --connect --host host --port port
nREPL 0.4.5
Clojure 1.9.0
Java HotSpot(TM) 64-Bit Server VM 10.0.1+10
user=> (+ 1 2)
3

Most users, however, are advised to use REPL-y or their favourite editor instead for optimal results.

Talking to an nREPL endpoint programmatically

If you want to connect to an nREPL server using the default transport, something like this will work:

user=> (require '[nrepl.core :as repl])
nil
user=> (with-open [conn (repl/connect :port 59258)]
     (-> (repl/client conn 1000)    ; message receive timeout required
       (repl/message {:op "eval" :code "(+ 2 3)"})
       repl/response-values))

If your nREPL server is running on a different machine or listening on a specific address different than the default one, you can use the :host keyword in the connect function to specify which address to connect to. E.g., to connect to a nREPL server listening on address 172.18.0.5 and port 4001:

=> (with-open [conn (repl/connect :host "172.18.0.5" :port 4001)]
     (-> (repl/client conn 1000)    ; message receive timeout required
       (repl/message {:op "eval" :code "(+ 2 3)"})
       repl/response-values))

response-values will return only the values of evaluated expressions, read from their (by default) pr-encoded representations via read. You can see the full content of message responses easily:

=> (with-open [conn (repl/connect :port 59258)]
     (-> (repl/client conn 1000)
       (repl/message {:op :eval :code "(time (reduce + (range 1e6)))"})
       doall      ;; `message` and `client-session` all return lazy seqs
       pprint))
nil
({:out "\"Elapsed time: 68.032 msecs\"\n",
  :session "2ba81681-5093-4262-81c5-edddad573201",
  :id "3124d886-7a5d-4c1e-9fc3-2946b1b3cfaa"}
 {:ns "user",
  :value "499999500000",
  :session "2ba81681-5093-4262-81c5-edddad573201",
  :id "3124d886-7a5d-4c1e-9fc3-2946b1b3cfaa"}
 {:status ["done"],
  :session "2ba81681-5093-4262-81c5-edddad573201",
  :id "3124d886-7a5d-4c1e-9fc3-2946b1b3cfaa"})

Each message must contain at least an :op (or "op") slot, which specifies the "type" of the operation to be performed. The operations supported by an nREPL endpoint are determined by the handlers and middleware stack used when starting that endpoint; the default middleware stack (described below) supports a particular set of operations, [detailed here](ops.md).

Embedding nREPL, starting a server

If your project uses Leiningen (v2 or higher), you already have access to an nREPL server for your project via lein repl (or, lein repl :headless if you don’t need the REPL-y terminal-based nREPL client to connect to the resulting nREPL server).

Otherwise, it can be extremely useful to have your application host a REPL server whereever it might be deployed; this can greatly simplify debugging, sanity-checking, panicked code patching, and so on.

nREPL provides a socket-based server that you can trivially start from your application. Add it to your project’s dependencies, and add code like this to your app:

=> (use '[nrepl.server :only (start-server stop-server)])
nil
=> (defonce server (start-server :port 7888))
='user/server

If you want your nREPL server to listen on a particular address instead of the default one, you can use the :bind keyword to specify the address to listen on. E.g., to make the nREPL server listen on address 172.18.0.5 and port 4001:

=> (use '[nrepl.server :only (start-server stop-server)])
nil
=> (defonce server (start-server :bind "172.18.0.5" :port 4001))
='user/server

Depending on what the lifecycle of your application is, whether you want to be able to easily restart the server, etc., you might want to put the value start-server returns into an atom or somesuch. Anyway, once your app is running an nREPL server, you can connect to it from a tool like Leiningen or Counterclockwise or REPL-y, or from another Clojure process, as shown in Connecting to an nREPL server.

You can stop the server with (stop-server server).

Server options

Note that nREPL is not limited to its default messaging protocol, nor to its default use of sockets. nREPL provides a transport abstraction for implementing support for alternative protocols and connection methods. Alternative transport implementations are available, and implementing your own is not difficult; read more about transports here.

Hot-loading dependencies

From time to time you’d want to experiment with some library without adding it as a dependency of your project. You can easily achieve this with tools.deps or pomegranate. Let’s start with a tools.deps example:

clj -Sdeps '{:deps {nrepl {:mvn/version "0.4.5"} org.clojure/tools.deps.alpha
                {:git/url "https://github.com/clojure/tools.deps.alpha.git"
                 :sha "d492e97259c013ba401c5238842cd3445839d020"}}}' -m nrepl.cmdline --interactive
network-repl
Clojure 1.9.0
user=> (use 'clojure.tools.deps.alpha.repl)
nil
user=> (add-lib 'org.clojure/core.memoize {:mvn/version "0.7.1"})
true
user=> (require 'clojure.core.memoize)
nil
user=>

Alternatively with pomegranate you can do the following:

❯ clj -Sdeps '{:deps {nrepl {:mvn/version "0.4.5"} com.cemerick/pomegranate {:mvn/version "1.0.0"}}}' -m nrepl.cmdline --interactive
network-repl
Clojure 1.9.0
user=> (use '[cemerick.pomegranate :only (add-dependencies)])
nil
user=> (add-dependencies :coordinates '[[org.clojure/core.memoize "0.7.1"]]
                         :repositories (merge cemerick.pomegranate.aether/maven-central
                                             {"clojars" "https://clojars.org/repo"}))
{[org.clojure/core.memoize "0.7.1"] #{[org.clojure/core.cache "0.7.1"] [org.clojure/clojure "1.6.0"]}, [org.clojure/core.cache "0.7.1"] #{[org.clojure/data.priority-map "0.0.7"]}, [org.clojure/data.priority-map "0.0.7"] nil, [org.clojure/clojure "1.6.0"] nil}
user=> (require 'clojure.core.memoize)
nil