We all know that we can start a web server in the REPL and develop a web app as interactively as any other app, we know how to connect to a remote Lisp image by starting a Swank server and how to interact with it from our favorite editor on our machine, we know we can build a self-contained binary of the web app and simply run it, but one thing I had not realized, despite being the basics, is that by starting the web app with sbcl --load app.lisp, we are dropped into the regular Lisp REPL, with the web server running in its own thread (as in development mode, but unlike with the binary), and that we can consequently interact with the running app.

As a demonstration, you can clone this repository and run the example like this:

* rlwrap sbcl --load run.lisp

This is SBCL 1.4.5.debian, an implementation of ANSI Common Lisp.
re information about SBCL is available at <http://www.sbcl.org/>.
[…]
; Loading "web-live-reload"
..................................................
Starting the web server on port 7890
Ready. You can access the application!
*

it will load project.asd, install 3 Quicklisp dependencies (you must have Quicklisp installed), start Hunchentoot on port 7890, and drop us into a REPL.

You’ll get this:

The template prints the *config* variable, which you can change in the REPL:

* (in-package :web-live-reload)
* (setf *config*
    '((:key "Name"
       :val "James")
      (:key "phone"
       :val "0098 007")
      (:key "secret language?"
       :val "Lisp")))

refresh, and voilà, your new config is live.

For functions, it is just the same (redefine the function fn that returns a string, if you want to try).

If a file changes (for example after a git pull), compile it with a usual load: (load "src/web.lisp").

You can also reload all the app with (ql:quickload :myproject), which will install the dependencies, without needing to restart the running image.

I was looking for a way to reload a user’s config and personal data from a running website, and this has proved very practical. I have no downtime, it is pure Lisp, it is the workflow I am used to. I am more cautious on using this to recompile the whole app, even though I did it without glitches so far. The thing to not do is to change the global state manually, aka to develop in production!

That’s all, but that made my day.

Bonus points:

  • after a git pull, the (Djula) templates are automatically updated. No operation is needed to see them live. (you can disable this by pushing :djula-prod into the features set)
  • you’ll understand and appreciate the difference between defparameter and defvar. Imagine you declare a variable with (defparameter *data* nil) and you populate it with some heavy computation at the application startup. Now if you load the file this declaration is in, you’ll set the data back to nil. If you declare it with defvar, you can live-reload your app and the data doesn’t go away. You can try both cases with the *config* variable.
  • the app started a Swank server on port 4006, if you want to try on your VPS.

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