Three main principles drive the design of Parrot ”speed, abstraction, and stability.
Speed is a paramount concern. Parrot absolutely must be as fast as possible, since the engine effectively imposes an upper limit on the speed of any program running on it. It doesn't matter how efficient your program is or how clever your program's algorithms are if the engine it runs on limps along. While Parrot can't make a poorly written program run fast, it could make a well-written program run slowly, a possibility we find entirely unacceptable.
Speed encompasses more than just raw execution time. It extends to resource usage. It's irrelevant how fast the engine can run through its bytecode if it uses so much memory in the process that the system spends half its time swapping to disk. While we're not averse to using resources to gain speed benefits, we try not to use more than we need, and to share what we do use.
Abstraction indicates that things are designed such that there's a limit to what anyone needs to keep in their head at any one time. This is very important because Parrot is conceptually very large, as you'll see when you read the rest of the chapter. There's a lot going on, too much to keep the whole thing in mind at once. The design is such that you don't have to remember what everything does, and how it all works. This is true regardless of whether you're writing code that runs on top of Parrot or working on one of its internal subsystems.
Parrot also uses abstraction boundaries as places to cheat for speed. As long as it looks like an abstraction is being completely fulfilled, it doesn't matter if it actually is being fulfilled, something we take advantage of in many places within the engine. For example, variables are required to be able to return a string representation of themselves , and each variable type has a "give me your string representation" function we can call. That lets each class have custom stringification code, optimized for that particular type. The engine has no idea what goes on beneath the covers and doesn't care ”it just knows to call that function when it needs the string value of a variable.
Stability is important for a number of reasons. We're building the Parrot engine to be a good backend for many language compilers to target. We must maintain a stable interface so compiled programs can continue to run as time goes by. We're also working hard to make Parrot a good interpreter for embedded languages, so we must have a stable interface exposed to anyone who wants to embed us. Finally, we want to avoid some of the problems that Perl 5 has had over the years that forced C extensions written to be recompiled after an upgrade. Recompiling C extensions is annoying during the upgrade and potentially fraught with danger. Such backward-incompatible changes have sometimes been made to Perl itself.