The Obarray

Early Lisps interned all their symbols in a single symbol table called the obarray. Every program you loaded into your Lisp image would share the obarray. Memory was limited, so you usually only ran one program (like Macsyma) at a time.

But as memory got larger and cheaper, people started to want to run multiple Lisp programs, like Macsyma and Emacs, at the same time. The problem was they would collide over the use of the symbols. (In particular, over the property lists.) Someone — I’m not sure who — came up with a hack that would swap out the obarray depending on which program you were loading.

The origin of the package system was "the obarray hack". Packages are first-class named obarrays, with some support for controlling the sharing of symbols among the obarrays, a limited form of inheritance and some code that maintains consistency.

In any but the smallest Lisp project, you need to decide on a symbol and package strategy. Some people keep with the original spirit of the package system and create just a handful of coarse-grained packages that each encompass a logical program. Other people use packages as modules, which gives you a set of many fine-grained packages, one to each module in your system.

I favor the former approach. I either use one package for my entire program, or I break it into just a couple of main packages. I don’t try for a finer grained approched. The package system wasn’t really designed for a fine-grained appoach.

Screwing up your packages can easily make your system unusable. If you dynamically create and link packages as you load your code, you have to be careful about the order in which you load your files. Load a file out of order and you’ll end up with dozens of symbols interned in the wrong package.

When I’m working on a project, I always have a file called package.lisp which defines all the packages in the project in one place. The package.lisp is always the first file loaded. Once I’ve done this, then the order in which the other files are loaded becomes far less important. It saves a lot of headaches.