1. Lua Patient Zero: Grim Fandango Bret Mogilefsky Formerly of LucasArts, now SCEA

2. Setting the stage One famous designer: Tim Schafer One venerable game engine: SCUMM Huge pedigree of beloved games includedManiac Mansion, Secret of Monkey Island, Sam and Max, Day of the Tentacle, Full Throttle One new game engine: Sith, for Jedi Knight One naïve coder: Me, fresh out of Cal! I stole from everyone!

3. Scripting not a new idea atLucasArts SCUMM dated back to 1986 Example: actor guybrush walk-to banana-tree wait-for-actoractor guybrush say-line “Mmm, bananas…” wait-for-actor actor guybrush face-camera actor guybrush say-line “Wish I had a banana-picker” Various attempts to unseat it had failed Nobody stopped me, though they predicted doom

4. Breaking with the past SCUMM was long in the tooth and newb-hostile SCUMM the language was inseparable from SCUMM the engine I am as guilty of not-invented-here as anyone Got sets, characters, lighting, animation, and audio in no time, but no scripting Jedi Knight put me way ahead, but it wasn’t an adventure game

5. Why Lua I was lucky! <Insert pic of Dr. Dobb’s Journal> It was small, elegant, and ready to go The manual was small enough that you could hand it to someone and expect them to read it It had no external dependencies The embedding API was ideal The code itself was approachable and readable

6. Lua as I found it Started with Lua 2.5 Switched to Lua 3.1 beta before shipping Very nice …but still lacked coroutines Any LEC adventure game required them!

7. Why coroutines? Independent characters and objects get programmed with isolated scripts controlling their actions Think about what it would be like in C/C++! Threads were too heavy for undead pigeons, flaming beavers, driving demons, and Pentium 90MHz processors True concurrency wasn’t needed; just wanted to pick up where we left off last time each frame

8. Lua modifications Addition of coroutines Made it re-entrant Flattened the Lua stack Implemented dump/restore of Lua state

9. Lua re-entrancy Took state out of global variables to make it thread-safe First attempt was an array of Lua state structures looped over until a “yield()” was called, using a single thread But… DISASTER!

10. Lua’s recursive stack Every Lua call also pushed a stack frame in C A yield from one Lua instance to resume another might happen at a different stack depth Welcome to Crash City <Sequence diagram: C->Lua->C->Lua->C->Lua>

11. Flattening Lua’s stack Instead of recursing, return a yield code, push a structure, and start the next function <C->Lua->Lua->Lua> A couple years later, “Stackless Python” solved the same problem It still exists, whereas Lua incorporated coroutines as part of the language This would not have been possible if Lua wasn’t nice code to begin with

13. Command-line and console Copy and paste code into game on-the-fly Iterate on a bugfix without restarting Trigger any condition as needed This kind of productivity was unprecedented! The compiler was small so we left it in

14. Debugger Written by a scripter using the in-language line-hook and call-hook facility Added the task-hook for our coroutines Fancy features like evaluating expressions at each step were trivial due to compiler in engine

15. Tool macro language Developed choreography rehearsal tool alongside game Stitched together assets into “costumes” and coordinated timing ChoreTool used Lua as a macro language Example: Tie real game trigger logic to a menu item Example: Automate insertion of keys at regular intervals Some macros became permanent parts of the tool, saving lots of programmer time

16. Patching Blocker bugs in scripts were trivially replaced Saved enormous amounts of burn time Update to shipped game retro-patched save games as well Probably not an issue if we’d saved differently

17. It wasn’t all roses High memory requirements Debugger went stale Heavyweight garbage collection So much flexibility it was easy to write bugs Typos in variable names not caught Lacked languageconventions No example code to examine Savegames steadily grew larger as you progress…

18. How’d we do? Despite huge technical risks, we pulled it off Great reviews, and the game remains beloved Poor timing: We lost to Half-Life in “game of the year” categories over and over The engine went on to ship Escape from Monkey Island (with lots of mods) But really, the end of the adventure genre was upon us, and that was the end of the line for GrimE

19. TheLua explosion Concurrent success of QuakeC and UnrealScript had made scripting languages HOT! GDC99 held a panel on them Rob Huebner on Java in Vampire: The Masquerade Kevin Bruner on writing your own for Obi-Wan Seamus McNally on not using one for Treadmarks 200 miserable developers slumped in their chairs “Or you could just use Lua…” Furious scribbling followed by sudden influx on the mailing list

20. Why Lua endures It’s small, simple, and proven Lua turned “scripting language” into a commodity feature You would be crazy to write your own and expect to do better