Sly: Introduction

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1 Introduction

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Sly is a 2D/3D game engine written in the GNU Guile dialect of the +Scheme programming language. With the help of Guile, Sly provides all +of the essential building blocks for making video games, such as: +window management, input even handling, rendering, linear algebra, and +powerful scripting capabilities. Sly differentiates itself from +traditional game engines by providing a dynamic live codinng +environment and a functional API. +

For those unfamiliar with the term, “live coding” is the practice of +improvised interactive programming. Sly provides a suitable +environment for live coding by leveraging Guile’s cooperative REPL +server. When used with a powerful editing tool such as Emacs (with +the fantastic Geiser extension), programmers may evaluate arbitrary +code and see the effects of their modifications in real time. This +tight feedback loop allows for faster prototyping, greater +productivity, and most importantly, more fun. +

Functional reactive programming (FRP) is a technique used to model +time-varying values with pure functions. A pure function is a +function whose return value depends solely upon its arguments. They +also produce no side-effects, such as calling ’set!’ on a variable or +writing to a file on disk. Sly encapsulates time-varying values in +“signals”, a high-level data structure for controlling the flow of +events. Unlike imperative event callbacks, signals can easily be +composed to form new signals. By modeling game state with pure +functions and immutable data, a game can be seen as a function of +time. To play the game is to “fold” (accumulate a result) over +time. Constructing a game this way allows for deterministic behavior +that is easier to reason about and test than the accumulation of +side-effects seen in traditional game engines. The signal interface +is also declarative, meaning that the programmer describes what +the state of the world should be like at any given time, rather than +how to get there. +

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