Gerbil Development with Emacs

The natural home for Gerbil development is Emacs, the ultimate parenthesis manipulation machine.

Standard Emacs Setup

Gerbil comes with a custom editing mode, which extends scheme-mode with font-lock and indentation for Gerbil code: gerbil-mode.el. See below for additional functionality provided by gerbil-mode.

You can add it to your autoload path (eg by linking in $HOME/.emacs.d) and adding this to your .emacs:

(autoload 'gerbil-mode "gerbil-mode" "Gerbil editing mode." t)

You should further utilize Gambit's inferior mode, as it offers debugger integration with sources on emacs. By default it is installed in $GERBIL_INSTALL_PREFIX/share/emacs/site-lisp/gambit.el alongside gerbil-mode.el.

You can add it to your autoload path and then add this to your .emacs:

(require 'gambit)
(add-hook 'inferior-scheme-mode-hook 'gambit-inferior-mode)

You can then make gxi your scheme program by setting scheme-program-name:

(defvar gerbil-program-name
  (expand-file-name "/opt/gerbil/bin/gxi")) ; default installation, adjust for your GERBIL_INSTALL_PREFIX
(setq scheme-program-name gerbil-program-name)

And you can now run Gerbil with M-x run-scheme.

Note that both gerbil-mode.el and gambit.el are installed by default in $GERBIL_INSTAL_PREFIX/share/emacs/site-lisp when installing gerbil.

Doom Emacs Setup

Gerbil is easy to setup in Doom Emacs. Open Doom Emacs. Ensure you are at the Doom Emacs splash creeen. Type SPC f P and choose packages.el. This will open the Doom Emacs package configuration file.

To add Gambit and Gerbil modes, go to the bottom of package.el and add the following:

(package! gambit :recipe
  (:host github
   :repo "gambit/gambit"
   :files ("misc/gambit.el")
   :branch "master"))

(package! gerbil-mode :recipe
  (:host github
   :repo "mighty-gerbils/gerbil"
   :files ("etc/gerbil-mode.el")
   :branch "master"))

Save the file with :w and close the buffer with SPC b d. You should arrive back at the Doom Emacs splash screen.

In order to fetch and install these packaged you must first exit doom emacs using SPC q K and then run doom sync.

Let's create a convenient key sequence that opens a Gerbil REPL in a nice window layout. Type SPC f P again and choose config.el. This opens the private configuration file for your Doom Emacs installation. Go to the bottom of config.el and add the following:

(defun gerbil-setup-buffers ()
    "Change current buffer mode to gerbil-mode and start a REPL"
    (shrink-window-horizontally 2)
    (let ((buf (buffer-name)))
      (other-window 1)
      (run-scheme "gxi")
      (switch-to-buffer-other-window "*scheme*" nil)
      (switch-to-buffer buf)))

  (global-set-key (kbd "C-c C-g") 'gerbil-setup-buffers)

Save the file with :w and close the buffer with SPC b d. You should arrive back at the Doom Emacs splash screen.

Let's set up some built-in Doom Emacs initializations that can make our life easier. Emacs is a parentheses generating machine. Let's make some changes for Gerbil programming in evil mode. Type SPC f P and select init.el. To load a built-in Doom Emacs package, just uncomment it. For Gerbil programming, we recommend:

  • scheme
  • (default +bindings +smartparens)

After you enable the built-in packages you want by uncommenting, save the init.el with :w and close the buffer with SPC b d. You should be back at the Doom Emacs spash screen.

In evil mode, you can get key conflicts with the paranthesis handling in lisp and scheme modes. Let's fix that. Type SPC f P and select packages.el again. Add the following at the end of the file:

(package! evil-cleverparens)

evil-cleverparens removes the key conflicts in evil mode. It uses functions in the smartparens package, which is why we previously enabled it in init.el.

That's it. You should be ready to plot and scheme with an apocalyptic Doom Emacs. Your Gerbil programs will be pleased.


Gerbil comes with a tool to build emacs tags from Gerbil sources, called gxtags.

By default, the install script installs all the sources in GERBIL_INSTALL_PREFIX/src; so it is matter of running the tool to create a TAGS index for your Gerbil installation. You can do this by running gxtags ${GERBIL_INSTALL_PREFIX}/src. This tags table contains all exported symbols for the entire Gerbil distribution and thus allows you to easily browse and auto-complete Gerbil code and explore the system.

You can load the tags table with M-x visit-tags-table and selecting the tags file. If you want it to be a permanent part of your editing experience, you can add path/to/TAGSS to your tags table list by adding this to your .emacs:

(visit-tags-table "path/to/TAGS")

The Gerbil package manager also generates a TAGS table for all installed packages; by default this lives in ~/.gerbil/pkg/TAGS. You can add this to your emacs tags table list with

(visit-tags-table "~/.gerbil/pkg/TAGS")

You should also generate tags for your own code by using gxtags. The invocation is very simple:

gxtags [-a] [-t emacs|vim] [-o TAGS] source-file-or-directory ...

Note that the tool can also create a TAGS index in vim format, by using the -t vim option or setting the GERBIL_TAGS_FORMAT=vim environment variable.

Gerbil Mode Interactive Functions

Aside from indentation and fontification, gerbil-mode provides a handful of interactive functions useful for Gerbil development:

M-x gerbil-import-current-buffer
M-x gerbil-reload-current-buffer
M-x gerbil-compile-current-buffer

These functions import, reload, and compile the source module visited by the current buffer respectively. They link syntax errors back to the source by popping the offending buffer at the point of offense.

M-x gerbil-build

This function builds the current Gerbil source tree by invoking the first script discovered by walking up the directory hierarchy.

SLIME Integration

There is preliminary support for SLIME using ecraven's r7rs-swank.

See drewc's guide for how to set it up.

Note that this is being phased out as we plan to implement a Gerbil LSP server for the next Gerbil release (v0.19).

Use-Package Example Configuration

Example use-package definition to get you hacking in no time. All you have to do is to havegxiin your path and copy the code snippet below into your Emacs config.

  (defvar *gerbil-path*
    (shell-command-to-string "gxi -e '(display (path-expand \"~~\"))'\
      -e '(flush-output-port)'"))

  (use-package gerbil-mode
    :when (file-directory-p *gerbil-path*)
    :ensure nil
    :straight nil
    :defer t
    :mode (("\\.ss\\'"  . gerbil-mode)
           ("\\.pkg\\'" . gerbil-mode))
    :bind (:map comint-mode-map
		(("C-S-n" . comint-next-input)
		 ("C-S-p" . comint-previous-input)
		 ("C-S-l" . clear-comint-buffer))
		:map gerbil-mode-map
		(("C-S-l" . clear-comint-buffer)))
    (autoload 'gerbil-mode
      (expand-file-name "share/emacs/site-lisp/gerbil-mode.el" *gerbil-path*)
      "Gerbil editing mode." t)
    ((gerbil-mode-hook . linum-mode)
     (inferior-scheme-mode-hook . gambit-inferior-mode))
    (require 'gambit
             (expand-file-name "share/emacs/site-lisp/gambit.el" *gerbil-path*))
    (setf scheme-program-name (expand-file-name "bin/gxi" *gerbil-path*))

    (let ((tags (locate-dominating-file default-directory "TAGS")))
      (when tags (visit-tags-table tags)))
    (let ((tags (expand-file-name "src/TAGS" *gerbil-path*)))
      (when (file-exists-p tags) (visit-tags-table tags)))

    (when (package-installed-p 'smartparens)
      (sp-pair "'" nil :actions :rem)
      (sp-pair "`" nil :actions :rem))

    (defun clear-comint-buffer ()
      (with-current-buffer "*scheme*"
	(let ((comint-buffer-maximum-size 0))

  (defun gerbil-setup-buffers ()
    "Change current buffer mode to gerbil-mode and start a REPL"
    (shrink-window-horizontally 2)
    (let ((buf (buffer-name)))
      (other-window 1)
      (run-scheme "gxi")
      (switch-to-buffer-other-window "*scheme*" nil)
      (switch-to-buffer buf)))

  (global-set-key (kbd "C-c C-g") 'gerbil-setup-buffers))

To start open a Gerbil file or type C-c C-g. Alternatively run M-x gerbil-mode (to launch a REPL run-scheme).

gerbil-mode - functions worth mentioning:

C-x C-e  send sexp behind cursor to the REPL (scheme-send-last-sexp)
C-c C-c  send region to REPL                 (scheme-send-region)
C-c C-l  load file into REPL                 (scheme-load-file)
C-S-l    clear the comint buffer             (clear-comint-buffer)

inferior-scheme-mode - functions worth mentioning:

C-S-l    clear the comint buffer   (clear-comint-buffer)
C-c _    close extra pop-up buffer (gambit-kill-last-popup)
C-S-p    previous item in history  (comint-previous-input)
C-S-n    next item in history      (comint-next-input)

For more information read C-h f gerbil-mode.