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Think about Loose Coupling


by root (Scribe)
on Dec 23, 1999 at 00:49 UTC ( #1133=perlfunc: print w/ replies, xml ) Need Help??


See the current Perl documentation for lib:autouse.

Here is our local, out-dated (pre-5.6) version:

autouse - postpone load of modules until a function is used

  use autouse 'Carp' => qw(carp croak);
  carp "this carp was predeclared and autoused ";

If the module Module is already loaded, then the declaration

  use autouse 'Module' => qw(func1 func2($;$) Module::func3);

is equivalent to

  use Module qw(func1 func2);

if Module defines func2() with prototype ($;$), and func1() and func3() have no prototypes. (At least if Module uses Exporter's import, otherwise it is a fatal error.)

If the module Module is not loaded yet, then the above declaration declares functions func1() and func2() in the current package, and declares a function Module::func3(). When these functions are called, they load the package Module if needed, and substitute themselves with the correct definitions.


Using autouse will move important steps of your program's execution from compile time to runtime. This can

  • Break the execution of your program if the module you autoused has some initialization which it expects to be done early.

  • hide bugs in your code since important checks (like correctness of prototypes) is moved from compile time to runtime. In particular, if the prototype you specified on autouse line is wrong, you will not find it out until the corresponding function is executed. This will be very unfortunate for functions which are not always called (note that for such functions autouseing gives biggest win, for a workaround see below).

To alleviate the second problem (partially) it is advised to write your scripts like this:

  use Module;
  use autouse Module => qw(carp($) croak(&$));
  carp "this carp was predeclared and autoused ";

The first line ensures that the errors in your argument specification are found early. When you ship your application you should comment out the first line, since it makes the second one useless.


Ilya Zakharevich (



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