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Trident Mode

This is an Emacs minor mode and collection of commands for working with Parenscript code in SLIME and sending it to the browser via Skewer. The goal is to create an environment for hacking Parenscript which fits as naturally as possible into the Lisp style of interactive development.

There’s at least one other project with related goals, slime-proxy, though at the time of writing it’s unclear whether it’s still being actively developed.


Trident is available on MELPA, meaning a simple M-x package-install RET trident-mode RET will install both it and its dependencies.

To enable MELPA, if you haven’t already, add something like the following to your Emacs configuration:

(require 'package)
(add-to-list 'package-archives
             '("melpa" . "") t)

The dependencies that will be installed are:

Trident also requires a Common Lisp implementation and Parenscript. Quicklisp is the best way to install Parenscript.


To enable trident-mode in a SLIME buffer: M-x trident-mode.

To have lisp-mode, slime-mode, and trident-mode all enable automatically for any file with an extension of “.paren”:

(add-to-list 'auto-mode-alist (cons "\\.paren\\'" 'lisp-mode))
(add-hook 'lisp-mode-hook
          #'(lambda ()
              (when (and buffer-file-name
                         (string-match-p "\\.paren\\>" buffer-file-name))
                (unless (slime-connected-p)
                  (save-excursion (slime)))
                (trident-mode +1))))

Parenscript must be loaded in your Common Lisp image, and you’ll probably also want to import its symbols:

(ql:quickload :parenscript)
(use-package :parenscript)

With the above taken care of it’s time to skewer the browser. See Skewer’s README for detailed information on the multiple ways you can connect to a site - including sites on servers you don’t control.

The fastest way to simply try things out is to run M-x run-skewer. Skewer will load an empty page in your browser and connect to it. You can immediately begin using Trident’s evaluation commands (described below); to additionally open a JavaScript REPL you can run M-x skewer-repl.


Code expansion commands

These commands generate JavaScript from the Parenscript code and display it but don’t send it to the browser for evaluation:

  • trident-expand-sexp
  • trident-expand-last-expression
  • trident-expand-defun
  • trident-expand-region
  • trident-expand-buffer
  • trident-expand-dwim

From within an expansion buffer you can press e to send the JavaScript to the browser, w to copy it to the kill ring, s to save it to a file (you’ll be prompted for the destination) or q to dismiss the buffer. The copy command, w, acts on the region if it’s active or the entire buffer otherwise.

Additionally, you can use M-x trident-compile-buffer-to-file to expand the current buffer and save the generated code directly to a file.

Code evaluation commands

These commands first compile the Parenscript code to JavaScript and then immediately send to it the browser to be evaluated:

  • trident-eval-sexp
  • trident-eval-last-expression
  • trident-eval-defun
  • trident-eval-region
  • trident-eval-buffer
  • trident-eval-dwim

Key bindings

The traditional set of code evaluation key bindings is a poor fit for Trident, since they would shadow SLIME’s equivalent commands and that’s probably not what you want. That leaves us without a clear convention to follow, so by default we don’t establish any key bindings at all. However, the function trident-add-keys-with-prefix will add two-key key bindings for all commands behind a prefix of your choice.

For example:

(trident-add-keys-with-prefix "C-c C-e")
;; The key sequence for trident-eval-region is "e r", so it's now bound to "C-c
;; C-e er"

The full list of key bindings trident-add-keys-with-prefix will establish is:

  • e RETtrident-eval-sexp
  • e etrident-eval-last-expression
  • e dtrident-eval-defun
  • e rtrident-eval-region
  • e btrident-eval-buffer
  • e SPCtrident-eval-dwim
  • x RETtrident-expand-sexp
  • x etrident-expand-last-expression
  • x dtrident-expand-defun
  • x rtrident-expand-region
  • x btrident-expand-buffer
  • x SPCtrident-expand-dwim

Evaluation commands begin with an “e”, expansion commands with “x”. The second letter is generally mnemonic but not always. The -sexp commands use RET in correspondence to slime-expand-1, and the -dwim commands use the space bar because it’s easy and comfortable to hit.

Please consider these keys provisional, and let me know if you have any ideas for improving the arrangement.

If you really want to shadow SLIME’s key bindings in buffers where trident-mode is active you could do something like this:

(defun steal-slime-keys-for-trident! ()
  ;; Don't affect all SLIME buffers, just where invoked
  (make-local-variable 'slime-mode-map)
  (let ((map slime-mode-map))
    (define-key map (kbd "C-x C-e") nil)
    (define-key map (kbd "C-c C-r") nil)
    (define-key map (kbd "C-M-x")   nil)
    (define-key map (kbd "C-c C-k") nil)
    (define-key map (kbd "C-c C-m") nil))
  (let ((map trident-mode-map))
    (define-key map (kbd "C-x C-e") 'trident-eval-last-expression)
    (define-key map (kbd "C-c C-r") 'trident-eval-region)
    (define-key map (kbd "C-M-x")   'trident-eval-defun)
    (define-key map (kbd "C-c C-k") 'trident-eval-buffer)
    (define-key map (kbd "C-c C-m") 'trident-expand-sexp)))

(add-hook 'trident-mode-hook 'steal-slime-keys-for-trident!)

Other amenities

slime-selector is a great feature and Trident can optionally integrate with it. If you call trident-add-slime-selector-methods, two entries related to trident-mode will be added. One, invoked with p, will take you to the most recently visited buffer where trident-mode is active (excluding buffers which are already visible). The other, on P, will take you to a scratch buffer with trident-mode enabled, creating the buffer if necessary.

Speaking of the scratch buffer, the trident-scratch command will take you straight there.

Still do be done

  • Add some tests.
  • Better documentation.
  • Look into adding a REPL.
  • See if more integration with SLIME is possible.
  • Command(s) for compiling to a file.
  • Similar support for CL-WHO and/or CSS-LITE?
  • Add support for Customize.


Contributions are very welcome. Since I’ve just started working on this and don’t have everything figured out yet, please first contact me on GitHub or send me an email so we can talk before you start working on something.


Emacs minor mode for live Parenscript interaction







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