| If you supply a package, you need to follow some simple coding conventions to make your library easier to use by other programmers. You can use the namespace facility introduced in Tcl 8.0. You can also use conventions to avoid name conflicts with other library packages and the main application. This section describes the conventions I developed before namespaces were added to Tcl. A Module Prefix for Procedure Names The first convention is to choose an identifying prefix for the procedures in your package. For example, the preferences package in Chapter 42 uses Pref as its prefix. All the procedures provided by the library begin with Pref. This convention is extended to distinguish between private and exported procedures. An exported procedure has an underscore after its prefix, and it is acceptable to call this procedure from the main application or other library packages. Examples include Pref_Add, Pref_Init, and Pref_Dialog. A private procedure is meant for use only by the other procedures in the same package. Its name does not have the underscore. Examples include PrefDialogItem and PrefXres. This naming convention precludes casual names like doit, setup, layout, and so on. Without using namespaces, there is no way to hide procedure names, so you must maintain the naming convention for all procedures in a package. A Global Array for State Variables You should use the same prefix on the global variables used by your package. You can alter the capitalization; just keep the same prefix. I capitalize procedure names and use lowercase letters for variables. By sticking with the same prefix you identify what variables belong to the package and you avoid conflict with other packages.  | Collect state in a global array. |
In general, I try to use a single global array for a package. The array provides a convenient place to collect a set of related variables, much as a struct is used in C. For example, the preferences package uses the pref array to hold all its state information. It is also a good idea to keep the use of the array private. It is better coding practice to provide exported procedures than to let other modules access your data structures directly. This makes it easier to change the implementation of your package without affecting its clients. If you do need to export a few key variables from your module, use the underscore convention to distinguish exported variables. If you need more than one global variable, just stick with the prefix convention to avoid conflicts. The Official Tcl Style Guide John Ousterhout has published two programming style guides, one for C programming known as "The Engineering Manual" and one for Tcl scripts known as "The Style Guide". These describe details about file structure as well as naming conventions for modules, procedures, and variables. The Tcl Style Guide conventions use Tcl namespaces to separate packages. Namespaces automatically provide a way to avoid conflict between procedure names. Namespaces also support collections of variables without having to use arrays for grouping. You can find these style guides on the CD-ROM and also in ftp://ftp.scriptics.com/pub/tcl/doc. The Engineering Manual is distributed as a compressed tar file, engManual.tar.Z, that contains sample files as well as the main document. The Style Guide is distributed as styleGuide.ps (or .pdf). | 
