Because packages are used by the reader, a package must be defined before you can **LOAD** or **COMPILE-FILE** a file that contains an **IN-PACKAGE** expression switching to that package. Packages also must be defined before other **DEFPACKAGE** forms can refer to them. For instance, if you’re going to :use COM.GIGAMONKEYS.TEXT-DB in COM.GIGAMONKEYS.EMAIL-DB, then COM.GIGAMONKEYS.TEXT-DB‘s **DEFPACKAGE** must be evaluated before the **DEFPACKAGE** of COM.GIGAMONKEYS.EMAIL-DB.

The best first step toward making sure packages exist when they need to is to put all your s in files separate from the code that needs to be read in those packages. Some folks like to create a foo-package.lisp file for each individual package, and others create a single packages.lisp that contains all the **DEFPACKAGE** forms for a group of related packages. Either approach is reasonable, though the one-file-per-package approach also requires that you arrange to load the individual files in the right order according to the interpackage dependencies.

Doing these steps by hand will get tedious after a while. For simple programs you can automate the steps by writing a file, load.lisp, that contains the appropriate **LOAD** and **COMPILE-FILE** calls in the right order. Then you can just **LOAD** that file. For more complex programs you’ll want to use a system definition facility to manage loading and compiling files in the right order.12

The other key rule of thumb is that each file should contain exactly one **IN-PACKAGE** form, and it should be the first form in the file other than comments. Files containing **DEFPACKAGE** forms should start with (in-package "COMMON-LISP-USER"), and all other files should contain an **IN-PACKAGE** of one of your packages.

On the other hand, it’s fine to have multiple files read in the same package, each with an identical **IN-PACKAGE** form. It’s just a matter of how you like to organize your code.

The other bit of packaging mechanics has to do with how to name packages. Package names live in a flat namespace—package names are just strings, and different packages must have textually distinct names. Thus, you have to consider the possibility of conflicts between package names. If you’re using only packages you developed yourself, then you can probably get away with using short names for your packages. But if you’re planning to use third-party libraries or to publish your code for use by other programmers, then you need to follow a naming convention that will minimize the possibility of name collisions between different packages. Many Lispers these days are adopting Java-style names, like the ones used in this chapter, consisting of a reversed Internet domain name followed by a dot and a descriptive string.