if(3pm) Perl Programmers Reference Guide if(3pm)
NAME
if - "use" a Perl module if a condition holds
SYNOPSIS
use if CONDITION, "MODULE", ARGUMENTS;
no if CONDITION, "MODULE", ARGUMENTS;
DESCRIPTION
"use if"
The "if" module is used to conditionally load another module. The
construct:
use if CONDITION, "MODULE", ARGUMENTS;
... will load "MODULE" only if "CONDITION" evaluates to true; it has no
effect if "CONDITION" evaluates to false. (The module name, assuming
it contains at least one "::", must be quoted when 'use strict "subs";'
is in effect.) If the CONDITION does evaluate to true, then the above
line has the same effect as:
use MODULE ARGUMENTS;
For example, the Unicode::UCD module's charinfo function will use two
functions from Unicode::Normalize only if a certain condition is met:
use if defined &DynaLoader::boot_DynaLoader,
"Unicode::Normalize" => qw(getCombinClass NFD);
Suppose you wanted "ARGUMENTS" to be an empty list, i.e., to have the
effect of:
use MODULE ();
You can't do this with the "if" pragma; however, you can achieve
exactly this effect, at compile time, with:
BEGIN { require MODULE if CONDITION }
"no if"
The "no if" construct is mainly used to deactivate categories of
warnings when those categories would produce superfluous output under
specified versions of perl.
For example, the "redundant" category of warnings was introduced in
Perl-5.22. This warning flags certain instances of superfluous
arguments to "printf" and "sprintf". But if your code was running
warnings-free on earlier versions of perl and you don't care about
"redundant" warnings in more recent versions, you can call:
use warnings;
no if $] >= 5.022, q|warnings|, qw(redundant);
my $test = { fmt => "%s", args => [ qw( x y ) ] };
my $result = sprintf $test->{fmt}, @{$test->{args}};
The "no if" construct assumes that a module or pragma has correctly
implemented an unimport() method -- but most modules and pragmata have
not. That explains why the "no if" construct is of limited
applicability.
BUGS
The current implementation does not allow specification of the required
version of the module.
SEE ALSO
Module::Requires(3) can be used to conditionally load one or more
modules, with constraints based on the version of the module. Unlike
"if" though, Module::Requires is not a core module.
Module::Load::Conditional(3) provides a number of functions you can use
to query what modules are available, and then load one or more of them
at runtime.
The provide module from CPAN can be used to select one of several
possible modules to load based on the version of Perl that is running.
AUTHOR
Ilya Zakharevich <mailto:ilyaz@cpan.org>.
COPYRIGHT AND LICENCE
This software is copyright (c) 2002 by Ilya Zakharevich.
This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.
perl v5.38.2 2023-11-28 if(3pm)
perl 5.38.2 - Generated Sun Dec 15 15:18:45 CST 2024