In Perl, is it better to use a module than to require a file?

Question

Another question got me thinking about different methods of code reuse: use vs. require vs. do

I see a lot of posts here where the question centers around the use of require to load and execute code. This seems to me to be an obvious bad practice, but I haven't found any good resources on the issue that I can point people to.

perlfaq8 covers the difference between use and require, but it doesn't offer any advice in regard to preference (as of 5.10--in 5.8.8 there is a quick bit of advice in favor of use).

This topic seems to suffer from a lack of discussion. I have a few questions I'd like to see discussed:

1. What is the preferred method of code reuse in Perl?
   • use ModuleName;
   • require ModuleName;
   • require 'file.pl';
   • do 'file.pl';
2. What is the difference between require ModuleName and require "file.pl"?
3. Is it ever a good idea to use require "file.pl"? Why or why not?

Ivetteivetts

Did you mean require Module or maybe do "file" instead of require "file"? – Hellkite

@Michael, require "file" actually uses do "file" to process its arugment, but it will only operate on a given file one time. – Ivetteivetts

@daotoad: That's not what I was asking about. Typically do is used for *.pl files while require is used for *.pm files. The file extension doesn't matter, of course, but by convention *.pl hasn't meant "perl library" for a long time. I was trying to clarify if you were referring to the old usage or if the question should be modified to reflect the three ways of loading external code (use, require, and do). It would make more sense (to me) to include all three forms in the discussion. – Hellkite

@Michael, yes, I mean the old style usage. For whatever reason, I see more people using require on SO than I see using do. It does make sense to include the use of do in the question. If you have a good idea on the wording, please edit the question. The goal is to get a good resource to point people at, instead of rewriting the same response ("use modules instead") over and over. – Ivetteivetts

Answer 1

Standard practice is to use use most of the time, require occasionally, and do rarely.

do 'file' will execute file as a Perl script. It's almost like calling eval on the contents of the file; if you do the same file multiple times (e.g. in a loop) it will be parsed and evaluated each time which is unlikely to be what you want. The difference between do and eval is that do can't see lexical variables in the enclosing scope, which makes it safer. do is occasionally useful for simple tasks like processing a configuration file that's written in the form of Perl code.

require 'file' is like do 'file' except that it will only parse any particular file one time and will raise an exception if something goes wrong. (e.g. the file can't be found, it contains a syntax error, etc.) The automatic error checking makes it a good replacement for do 'file' but it's still only suited for the same simple uses.

The do 'file' and require 'file' forms are carryovers from days long past when the *.pl file extension meant "Perl Library." The modern way of reusing code in Perl is to organize it into modules. Calling something a "module" instead of a "library" is just semantics, but the words mean distinctly different things in Perl culture. A library is just a collection of subroutines; a module provides a namespace, making it far more suitable for reuse.

use Module is the normal way of using code from a module. Note that Module is the package name as a bareword and not a quoted string containing a file name. Perl handles the translation from a package name to a file name for you. use statements happen at compile time and throw an exception if they fail. This means that if a module your code depends on isn't available or fails to load the error will be apparent immediately. Additionally, use automatically calls the import() method of the module if it has one which can save you a little typing.

require Module is like use Module except that it happens at runtime and does not automatically call the module's import() method. Normally you want to use use to fail early and predictably, but sometimes require is better. For example, require can be used to delay the loading of large modules which are only occasionally required or to make a module optional. (i.e. use the module if it's available but fall back on something else or reduce functionality if it isn't.)

Strictly speaking, the only difference between require Module and require 'file' is that the first form triggers the automatic translation from a package name like Foo::Bar to a file name like Foo/Bar.pm while the latter form expects a filename to start with. By convention, though, the first form is used for loading modules while the second form is used for loading libraries.

Hellkite

Answer 2

There is a major preference for using use, because it happens at an earlier state BEGIN {} during compilation, and the errors tend to be propagated to the user at a more appropriate time. It also calls the sub import {} function which gives the caller control over the import process. This is something that is heavily used. You can get the same effect, by calling the specific namespace's import, but that requires you to know the name of the namespace, and the file, and to code the call to the subroutine... which is a lot more work. Conversely, use, just requires you to know the namespace, and then it requires the file with the matching namespace -- thus making the link between namespaces and files less of an conscious to the user.

Read perldoc -f use, and perldoc -f require, for more information. Per perldoc -f use:
use is the same as BEGIN { require Module; Module->import( LIST ); } Which is just a lot more ugly.

Kisor

Answer 3

The main difference is around import/export. use is preferred when you're making use of a module because it allows you to specify which routines you wish to import into your namespace :

use MyModule qw(foo bar baz); # allows foo(), bar() and baz() to be used

use MyModule qw(); # Requires explicit naming (e.g. MyModule::foo).

use also gives runs the module's import() procedure which is often used to set the module up.

See the perldoc for use for more detail.

Thine

Actually, they're one in the same. In your example MyModule::import() is accepting qw(foo bar baz) and doing the exportation itself. It is the same facility, Exporter, and Sub::Exporter just generate an import(). – Kisor

Answer 4

Using use to module will include module during compile time which increases speed but uses more memory, whereas using require module will include during run time. Requiring a module without using import when needed uses less memory but reduces speed.

Vaudeville