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-*- mode: org -*-
Sly is a free software game engine written in [[https://gnu.org/software/guile][Guile Scheme]]. It
provides an abstraction layer above SDL and OpenGL for common game
programming requirements such as:
- Scripting
- Animation
- Shaders
- Sprites
- Tilesets
- Keyboard/mouse/joystick input
Sly differentiates itself from most other game engines by encouraging
[[http://toplap.org/about/][live coding]] and emphasizing [[http://elm-lang.org/learn/What-is-FRP.elm][functional reactive programming]].
* Inspiration
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 [[http://love2d.org/][LÖVE]], [[http://pygame.org/][Pygame]], and [[http://pyglet.org/][Pyglet]].
Sly's reactive nature is heavily inspired by the [[http://elm-lang.org/][Elm]] programming
language.
* Example
Here is the simplest Sly application (so far).
#+BEGIN_SRC scheme
(use-modules (sly))
;; Create OpenGL context and do other setup.
(sly-init)
;; Load a GNU head sprite.
(define gnu
(load-sprite "gnu.png"))
;; The way we see the world.
(define camera
(2d-camera #:area (make-rect 0 0 640 480)))
;; View the sprite from the perspective of the camera.
(define-signal scene
(with-camera camera
(move (vector2 320 240) (render-sprite gnu))))
(with-window (make-window #:title "Hello, world!")
(run-game-loop scene))
#+END_SRC
* Features
** 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.
#+BEGIN_SRC scheme
(with-window (make-window #:title "Best Game Ever"
#:resolution #(640 480))
(run-game-loop))
#+END_SRC
** 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.
Example:
#+BEGIN_SRC scheme
(define-signal position
(signal-fold v+ (vector2 320 240)
(signal-map (lambda (v) (v* v 4))
(signal-sample 1 key-arrows))))
#+END_SRC
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 [[https://gnu.org/software/guile/manual/html_node/Cooperative-REPL-Servers.html][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 [[http://www.nongnu.org/geiser/][Geiser]]
extension for GNU Emacs.
*** Geiser
#+BEGIN_SRC fundamental
M-x connect-to-guile
#+END_SRC
Use the default host and port settings when prompted.
* Building
Sly uses the typical GNU build system. First run =autogen.sh= and
then do the usual incantations.
#+BEGIN_SRC sh
./autogen.sh
./configure
make
sudo make install
#+END_SRC
See =INSTALL.org= for more detailed installation instructions.
* Developing
Users of GNU Guix can quickly create a development environment by
running:
#+BEGIN_SRC sh
guix environment -l guix.scm
./bootstrap && ./configure \
--with-libgslcblas-prefix=$(guix build gsl)
#+END_SRC
* Running Examples
To run an example when Sly has been installed:
#+BEGIN_SRC sh
cd examples
guile simple.scm
#+END_SRC
To run an example without installing Sly (useful when developing):
#+BEGIN_SRC sh
cd examples
../pre-inst-env guile simple.scm
#+END_SRC
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!
* Dependencies
- GNU Guile >= 2.0.11
- [[http://www.gnu.org/software/guile-opengl/][guile-opengl]] >= 0.1.0
- [[https://dthompson.us/pages/software/guile-sdl2.html][guile-sdl2]] >= 0.1.0
- GNU Scientific Library (GSL)
* Releases
Releases can be found on Sly's [[http://dthompson.us/pages/software/sly.html][home page]].
* Community
For help and general discussion, join the =#sly= IRC channel on
irc.freenode.net.
* License
Sly is licensed under the GNU General Public License version 3 or
later.
See =COPYING= for the full license text.
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