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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-libfreeimage-prefix=$(guix build freeimage) \
        --with-libgslcblas-prefix=$(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://www.gnu.org/software/guile-sdl/index.html][guile-sdl]] >= 0.5.0
  - SDL 1.2
  - FreeImage >= 3.0
  - 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.