Garbage collection is a particularly cross-cutting implementation issue, and tends to interfere with all kinds of abstraction. You're essentially making a syste

Garbage collection is a particularly cross-cutting implementation issue, and tends to interfere with all kinds of abstraction. You're essentially making a system that knows which specific ways abstraction can be broken safely, so that you can defer resource management to it in the future. Retrofitting a GC is even harder. You can bolt on a conservative collector (like boehm-gc), sure, but that can lead to many unexpected performance issues (see e.g. http://timetobleed.com/6-line-eventmachine-bugfix-2x-faster-gc-1300-requestssec/ http://timetobleed.com/6-line-eventmachine-bugfix-2x-faster-...). Writing a virtual machine for a GC'd language and then trying to give it a portable, reasonably convenient C API convinced me that it's a significantly harder issue than most people realize. (Particularly after scrapping my third prototype!) The C API also impacts many things, and tends to bring up nasty edge cases in the GC. Lua seems to have settled on a design that works unusually well, and is worth studying. Still, Lua can safely cut corners that Python and Ruby cannot - they have threads, which also break a lot of assumptions.