ing languages are often part of IDEs (such as the one shown in Figure 1) that provide forms for quickly modifying these attributes. The scripting languages themselves, however, are fairly conventional. Many companies use traditional scripting languages such as Lua or Python for scripting. Even companies that design their own languages usually stick with traditional format and control structures. Little effort has been spent tailoring these scripting languages for games.

One of the major problems with traditional scripting languages is that the programmer must be explicitly aware of low-level processing issues that have little to do with gameplay. Performance is a classic example of such a low-level issue. Animation frame rate is so important to developers that they optimize by counting the number of multiplies or adds in their code. This type of analysis is beyond the skill of most designers, however. Furthermore, existing languages provide almost no tools to help designers improve script performance.

Designers must also take performance into account when creating content. If the game runs too slowly, they may be forced to reduce the number of objects in the game, which in turn can significantly alter the playing experience. This is what occurred when The Sims was ported to consoles. In this game, a player indirectly controls a character (Sim) by purchasing furniture

or other possessions for it. Each piece of furniture is scripted to advertise its capabilities to the Sim periodically. The Sim then compares these capabilities with its needs in order to determine its next action. Furniture does not exist in isolation, however; a couch in front of a television is much more versatile than one alone in a room. Therefore, pieces of furniture also periodically poll the other furniture in the room to update their capabilities. As each piece of furniture may communicate with other pieces of furniture, the cost of processing a room can grow quadratically with the number of objects in the room. When the title was ported to consoles, the performance issue became so pronounced that the designers had to introduce a “feng shui meter” to prevent players from filling rooms with too many possessions.

Game developers have many techniques available to them for improving performance. Spatial indexes are one popular way of handling interactions between game objects at less than quadratic cost. Parallel execution is another possibility; many games are embarrassingly parallel, and developers leverage this fact for multicore CPUs and distributed multiplayer environments. These techniques are beyond the skill of the typical game designer, however, and are left to the software engineers.

Another low-level issue with scripting languages is the lack of transac-

tion support for massively multiplayer games. Individual scripts are often executed concurrently, particularly in massively multiplayer games, so designers need some form of transaction to avoid inconsistent updates to the game state. Indeed, script-level concurrency violations are one of the major causes of bugs in multiplayer environments.

To make scripting easier for designers, we have to provide them with simple tools for addressing these low-level issues. None of these problems is really new; many programming languages have been developed over the years to address them, but most of these languages make programming more difficult, not easier. Fortunately, designers do not need an arbitrary scripting language; they just need a language that helps them write games.

from Patterns to
Language features

Despite these problems, games are being developed. Game developers have come up with many ideas that, if not complete solutions, do ameliorate the problems. These ideas typically come in the form of programming patterns that have proven over time to be successful. Though developers use these programming patterns in creating game behavior, the scripting languages usually do not support them explicitly. One of the reasons object-oriented programming languages have been so successful is that object-oriented programming patterns existed long before the languages that supported them. Similarly, by examining existing programming practices in game development, we can design scripting languages that require very little retraining of developers. The challenge in developing a scripting language is identifying those patterns and creating language features to support them most effectively.

 

figure 1. the neverwinter nights 2 toolset is an extensive iDe that allows users to create new content for the game.

the state-effect Pattern One popular pattern in game development is the state-effect pattern. Every game consists of a long-running simulation loop. The responsiveness of the game to player input depends entirely on the speed at which the simulation loop can be processed. In the state-effect pattern, each iteration of the simulation loop consists of two phases: effect and update. In the effect phase,