Chapter 12. Script Libraries and Packages

Collections of Tcl commands are kept in libraries and organized into packages. Tcl automatically loads libraries as an application uses their commands. Tcl commands discussed are: package, pkg_mkIndex, auto_mkindex, unknown, and tcl_findLibrary.

Libraries group useful sets of Tcl procedures so that they can be used by multiple applications. For example, you could use any of the code examples that come with this book by creating a script library and then directing your application to check in that library for missing procedures. One way to structure a large application is to have a short main script and a library of support scripts. The advantage of this approach is that not all the Tcl code needs to be loaded to start the application. Applications start up quickly, and as new features are accessed, the code that implements them is loaded automatically.

The Tcl package facility supports version numbers and has a provide/require model of use. Typically, each file in a library provides one package with a particular version number. Packages also work with shared object libraries that implement Tcl commands in compiled code, which are described in Chapter 44. A package can be provided by a combination of script files and object files. Applications specify which packages they require and the libraries are loaded automatically. The package facility is an alternative to the auto loading scheme used in earlier versions of Tcl. You can use either mechanism, and this chapter describes them both.

If you create a package you may wish to use the namespace facility to avoid conflicts between procedures and global variables used in different packages. Namespaces are the topic of Chapter 14. Before Tcl 8.0 you had to use your own conventions to avoid conflicts. This chapter explains a simple coding convention for large Tcl programs. I use this convention in exmh, a mail user interface that has grown from about 2,000 to over 35,000 lines of Tcl code. A majority of the code has been contributed by the exmh user community. Such growth might not have been possible without coding conventions.