Functional reactive game programming in GNU Guile Scheme. (mirror)
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-*- mode: org -*-

Sly is a free software game engine written in Guile Scheme. It provides an abstraction layer above SDL and OpenGL for common game programming requirements such as:

  • Animation
  • Shaders
  • Sprites
  • Tilesets
  • Scene graph
  • Keyboard/mouse/joystick input
  • Scripting

Sly differentiates itself from most other game engines by encouraging live coding and emphasizing functional reactive programming.


Every programming language should have a fun, easy to use game library. Guile is no exception. Sly draws its inspiration from easy-to-use libraries/engines such as LÖVE, Pygame, and Pyglet. Sly’s reactive nature is heavily inspired by the Elm programming language.


Here is the simplest Sly application (so far).

(use-modules (sly))

;; Create OpenGL context and do other setup.

;; The object to render.
(define model
  (model-move (vector2 320 240) (load-sprite "gnu.png")))

;; The way we see the world.
(define camera (orthographic-camera 640 480))

;; View the model from the perspective of the camera.
(define-signal scene
  (make-scene camera model))

(with-window (make-window #:title "Hello, world!")
  (run-game-loop scene))


The Game Loop

Sly’s game loop doesn’t tie drawing and updating together. Instead, updates happen on a fixed timestep (60 ticks per second by default) while drawing happens as many times as possible. A framerate indepedent loop mitigates slow down that the user might experience when updating the game takes longer than drawing a frame at the desired rate. Instead of slowing to a crawl, some frames are dropped and the loop tries to catch up on updates. Additionally, a fixed timestep allows for a deterministic simulation, unlike a variable timestep.

To start up the game loop, simply call (run-game-loop). It’s a good idea to set up the game window prior to starting the loop via the with-window form.

(with-window (make-window #:title "Best Game Ever"
                          #:resolution #(640 480))

Functional Reactive Programming

Game state is a function of time. The player’s score, the current stage, an enemy’s hit points, etc. all change in response to events that happen at discrete points in time. Typically, this means that a number of callback procedures are registered to respond to events which mutate the relevant data structures. However, this approach, while simple and effective, comes at the price of readability, comprehension, and expression. Instead of explicitly mutating data and entering “callback hell”, Sly abstracts and formalizes the process using a functional reactive programming style.

In Sly, time-varying values are called “signals”, and they are defined in a declarative and functional manner. Rather than describing the process of mutation procedurally, one describes the relationship between signals instead. Signal relationships are described in a functional style using signal-map, signal-fold, signal-filter, and others.


(define-signal position
  (signal-fold v+ (vector2 320 240)
               (signal-map (lambda (v) (v* v 4))
                           (signal-sample 1 key-arrows))))

This signal describes a relationship between the arrow keys on the keyboard and the position of the player. signal-sample is used to trigger a signal update upon every game tick that provides the current state of the arrow keys. key-arrows is a vector that maps to the current state of the arrow keys, allowing for 8 directional movement. This vector is then scaled 4x to make the player move faster. Finally, the scaled vector is added to the previous player position via signal-fold. The player’s position is at (320, 240) initially. As you can see, there are no callbacks and explicit mutation needed. Those details have been abstracted away, freeing the programmer to focus on more important things.

As an added bonus, signals adapt to changes in their environment when defined using the define-signal form. This means that a signal can be re-defined at the REPL and other dependent signals will take notice and re-evaluate themselves automagically.

REPL Driven Development

Guile’s read-eval-print-loop allows you to develop your game while it is running! This allows you to see in real-time what your changes do to the game without having to kill, recompile, and restart the program every time a change is made.

Sly integrates Guile’s cooperative REPL server into the game loop. To activate this feature, import the (sly repl) module and call (start-sly-repl). To connect to the REPL server, use the Geiser extension for GNU Emacs.


M-x connect-to-guile

Use the default host and port settings when prompted.


Sly uses the typical GNU build system. First run and then do the usual incantations.

sudo make install

See for more detailed installation instructions.


Users of GNU Guix can quickly create a development environment by running:

guix environment -l package.scm

Running Examples

To run an example when Sly has been installed:

cd examples
guile simple.scm

To run an example without installing Sly (useful when developing):

cd examples
../pre-inst-env guile simple.scm

To quit an example:

  • Close the window
  • Press the ESCAPE key

Using the Sandbox

If you want to quickly create a Sly environment and start experimenting, run ./pre-inst-env sandbox. It will import many useful modules, start a REPL server, open a window, and start the game loop. Simply connect to the REPL server and start hacking!


Sly supports GNU/Linux currently. OS X support is in the works, but there are problems with guile-sdl. See for more details.


  • GNU Guile >= 2.0.11
  • guile-opengl >= 0.1.0
  • guile-sdl >= 0.5.0
  • SDL 1.2
  • FreeImage >= 3.0
  • GNU Scientific Library (GSL)


Releases can be found on Sly’s home page.


For help and general discussion, join the #sly IRC channel on


Sly is licensed under the GNU General Public License version 3 or later.

See COPYING for the full license text.