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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 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.

       provide can be used to select one of several possible modules to load,
       based on what version of Perl 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 v5.28.1                      2018-11-01                           if(3pm)

perl 5.28.1 - Generated Mon Jan 14 18:43:53 CST 2019
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