NREPL

A networked REPL for Chicken Scheme, basically csi over a TCP socket. nrepl is intended to be used during development and is insecure by nature.

nrepl can be useful when it's complicated to launch your Chicken Scheme application, yet you want a REPL available. For Emacs users, it can replace your usual run-scheme interpreter, see below.

Requirements

• The srfi-18 egg.

API

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

Listen to TCP port port number and wait for incoming connections. The host and backlog parameters are passed to tcp-listen.

host defaults to "127.0.0.1" which will allow incoming connections from the local machine only. If you plan on exposing the REPL publicly, you can specify (nrepl 1234 #:port "0.0.0.0"). Note that this has major security drawbacks as a host can easily be compromised using a REPL.

(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. This procedure can be used in the optional spawn-procedure of nrepl.

It reports exceptions, ensures data is flushed and limits the print output to avoid flooding your nrepl session (so that (make-vector 10000) is safe).

Practical use

rlwrap

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

$ csi -R nrepl -P '(nrepl 1234)' &
$ rlwrap nc localhost 1234
;; nrepl on (csi -R nrepl -P (nrepl 1234))
#;> (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

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 like this:

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

If Emacs doesn't let you enter spaces, press 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 nrepl srfi-18 spiffy)

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

(thread-start!
 (lambda ()
   (vhost-map `((".*" . ,(lambda (c) (app c)))))
   (start-server)))

(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 only top-level symbols can be redefined.

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))
                    proc
                    (lambda () (mutex-unlock! main-mutex))))))

(thread-start!
 (lambda ()
   (nrepl 1234
          #:spawn (lambda ()
                    (thread-start!
                     (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)
  (loop))

nrepl in compiled code

nrepl also works inside a compiled program. However, sometimes modules disappear due to compiler optimizations. Lowering your optimization level to -O1, for example, can help.

In order to avoid spamming a REPL session with very long prints, it's advicable to issue (set! ##sys#repl-print-length-limit 2048) somewhere at startup. csi does sets this, so it's only necessary for compiled code.

Source code repository

You can find the source here.

Author

Kristian Lein-Mathisen

License

BSD