1. NREPL
    1. Requirements
    2. API
    3. Practical use
      1. Terminal users
      2. Emacs users
      3. Example HTTP-server work-flow
      4. Example CPU-intensive main thread
      5. nrepl in compiled code
      6. nrepl on Android
    4. Source code repository
    5. Author
    6. License

A networked REPL for Chicken Scheme v5. Each new incoming connection runs in a new srfi-18 thread.



[procedure] (nrepl port #!key host backlog spawn)

Listen to TCP port port number and (blockingly) wait for incoming connections. The host and backlog parameters are passed to tcp-listen. (spawn) is called for each incomming connection without arguments where current-input-port, current-output-port and current-error-port are bound to the TCP connection. spawn defaults to creating a new srfi-18 thread and printing a welcome message.

You can use tcp-addresses and tcp-port-numbers to find out where the new session is coming from.

nrepl will loop for accepting incomming connections unless spawn returns #f.

[procedure] (nrepl-loop #!key eval read print writeln)

Start a standard REPL-loop: print the prompt, read an s-expression, evaluate the expression, print the result and repeat. Exceptions are reported and data is flushed. This can be used inside the optionally supplied spawn-procedure above.

Practical use

Any source-code you send down a nrepl session will not be persisted anywhere. You can reset your program state by restarting your program which may be useful sometimes.

Terminal users

Editing code directly from nc localhost 1234 isn't pleasant. Luckily, rlwrap works along nrepl to improve this experience:

 ➤ rlwrap nc localhost 1234
;; nrepl on (csi -s example.scm)
#;1> (define (hello) (print "this will be in my history"))

rlwrap will also save your read-line history for the next invokation rlwrap nc localhost 1234 which is handy!

Emacs users

nrepl plays very nicely with Emacs! If you're used to running M-x run-scheme and sending source-code from buffers into your REPL, an nrepl endpoint can be used as a Scheme interpreter. You can specify that you want to use nrepl with a prefix. For example:

C-u M-x run-scheme RET nc localhost 1234

Note that telling Emacs that nc localhost 1234 is your Scheme interpreter is tricky because C-u M-x run-scheme might not let you enter spaces. You can enter spaces by pressing C-q before pressing space.

Example HTTP-server work-flow

A real-world use-case for nrepl might be something like the following. Let's make a simple hello-world HTTP server using spiffy.

(import spiffy nrepl)

(define (app c)
  (send-response body: "hello world\n"))

 (lambda ()
   (vhost-map `((".*" . ,(lambda (c) (app c)))))

(print "starting nrepl on port 1234")
(nrepl 1234)

Now spiffy runs on port 8080:

 ➤ curl localhost:8080
hello world

What's nice about this is that, since app is a top-level variable, it can be replaced from the REPL:

 ➤ rlwrap nc localhost 1234
;; nrepl on (csi -s example.scm)
#;1> (define (app c) (send-response body: "repl hijack!\n"))
#;1> ^C

Now spiffy will use our top-level app for its proceeding requests:

 ➤ curl localhost:8080
repl hijack!

Note that app must be wrapped in a lambda for this to work, because the REPL can only replace top-level variable definitions.

The implications of this can be quite dramatic in terms of work-flow. If you write your app in a REPL-friendly way like this, you can modify you program behaviour on-the-fly from the REPL and never have to restart your process and lose its state.

Example CPU-intensive main thread

nrepl can be used for live-coding interactive application such as games. Adding (thread-start! (lambda () (nrepl 1234))) usually Just Works, where you can redefine top-level function and game state on-the-fly.

However, if the game-loop is eating up a lot of scheduler-time, you may find that your REPL becomes unresponsive. A good way to fix this is to wrap both the REPL and the game-loop in a mutex. This has another advantage in that it will ensure your REPL will not interfere with game-state (or OpenGL state) during game-loop iteration.

;;; wrapping nrepl eval in a mutex for responsiveness
;;; and game-loop thread-safety. running this and then doing:
;;;     echo '(thread-sleep! 1)' | nc localhost 1234
;;; should pause the game-loop for 1 second
(import nrepl srfi-18 chicken.time)

(define with-main-mutex
  (let ((main-mutex (make-mutex)))
    (lambda (proc)
      (dynamic-wind (lambda () (mutex-lock! main-mutex))
                    (lambda () (mutex-unlock! main-mutex))))))

 (lambda ()
   (nrepl 1234
          #:spawn (lambda ()
                     (lambda ()
                       (nrepl-loop eval: (lambda (x) (with-main-mutex (lambda () (eval x)))))))))))

(define (game-step)
  (print* "\r"  (current-milliseconds) "   ")
  (thread-sleep! 0.05))

(let loop ()
  (with-main-mutex game-step)

nrepl in compiled code

nrepl also works inside a compiled program. However, sometimes modules disappear due to compiler optimizations.

nrepl on Android

nrepl has been used successfully on Android target hardware for remote interactive development. Check out this Android example project.

Source code repository

You can find the source here.


Kristian Lein-Mathisen at Adellica