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They can do this through the import mechanism, which defines the interface between the module and the application that uses it At the application end, we specify our requirements with the use or require statements, with which we can pass a list of symbols (often, but not necessarily, subroutine names) Conversely, at the module end, we define an import subroutine to control how we respond to import (or, from our point of view, export) requests The Exporter module provides one such import subroutine that handles most common cases for us Either way, the interface defined through the import mechanism abstracts the actual module code, making it easier to reuse the module, and minimizing the chances of an application breaking if we made changes to the module.

Perl provides a number of special blocks that can be used in any Perl code but which are particularly useful for packages, and accordingly we spend some time discussing them in this chapter The BEGIN, END, INIT, and CHECK blocks allow a module to define initialization and cleanup code to be automatically executed at key points during the lifetime of the module The AUTOLOAD subroutine permits a package to react to unknown subroutine calls and stand in for them..

This can have serious side effects, such as accidentally putting in code that was taken out previously..

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A package declaration is the naming of a new namespace in which further declarations of variables and subroutines are placed. A module is simply a library file that contains such a declaration and an associated collection of subroutines and variables. The link between package name and module file would therefore appear to be a strong one, but this is not necessarily true. As we saw in the last chapter, the module that is loaded corresponds to the named package, but this does not imply that the module actually defines anything in that package. As an example, many of Perl s pragmatic modules are purely concerned with compile-time semantics and do not contribute anything new to the symbol table. In fact, a module doesn t have to include a package declaration at all. Any constructs it creates will simply be put into whatever package was in effect at the time it was loaded main, if none has been declared yet. However, this is unusual. By including a package definition we are able to use many different modules without worrying about clashes between similarly named definitions. The notable exception are libraries that include several modules from one master module; the master

defines the namespace and then loads all its children, none of which define a namespace and so place their definitions into the parent s namespace Similarly, the package declarations in a module don t have to correspond to the name of the module supplied to the use statement they just usually do In some cases, we might use a different package or define symbols in more than one package at the same time Since a module file usually only contains declarations of subroutines and variables, rather than code that actually does something, executing it has no visible effect However, subroutines and package variables are added to the symbol table, usually under the namespace defined by the package declaration Whatever else it does, in order for a module to be loaded successfully by either require or use, it must return a true value.

Unless the module actually contains a return statement outside a subroutine definition, this must be the last statement in the file Since a typical module contains mainly subroutine definitions (which don t return anything), we usually need to add an explicit return value to let Perl know that the module is happy We can also add code for initialization that does do something actively and have that return a conditional value This means we can, for example, programmatically have a module fail compilation if, say, an essential resource like a configuration file that it needs is missing Most modules do not have any initialization code to return a value, so in general we satisfy Perl by appending a 1 or in fact any true value to the end of the module file.

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