package if; $VERSION = '0.0608'; sub work { my $method = shift() ? 'import' : 'unimport'; unless (@_ >= 2) { my $type = ($method eq 'import') ? 'use' : 'no'; die "Too few arguments to '$type if' (some code returning an empty list in list context?)" } return unless shift; # CONDITION my $p = $_[0]; # PACKAGE (my $file = "$p.pm") =~ s!::!/!g; require $file; # Works even if $_[0] is a keyword (like open) my $m = $p->can($method); goto &$m if $m; } sub import { shift; unshift @_, 1; goto &work } sub unimport { shift; unshift @_, 0; goto &work } 1; __END__ =head1 NAME if - C<use> a Perl module if a condition holds =head1 SYNOPSIS use if CONDITION, "MODULE", ARGUMENTS; no if CONDITION, "MODULE", ARGUMENTS; =head1 DESCRIPTION =head2 C<use if> The C<if> module is used to conditionally load another module. The construct: use if CONDITION, "MODULE", ARGUMENTS; ... will load C<MODULE> only if C<CONDITION> evaluates to true; it has no effect if C<CONDITION> evaluates to false. (The module name, assuming it contains at least one C<::>, must be quoted when C<'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 F<Unicode::UCD> module's F<charinfo> function will use two functions from F<Unicode::Normalize> only if a certain condition is met: use if defined &DynaLoader::boot_DynaLoader, "Unicode::Normalize" => qw(getCombinClass NFD); Suppose you wanted C<ARGUMENTS> to be an empty list, I<i.e.>, to have the effect of: use MODULE (); You can't do this with the C<if> pragma; however, you can achieve exactly this effect, at compile time, with: BEGIN { require MODULE if CONDITION } =head2 C<no if> The C<no if> construct is mainly used to deactivate categories of warnings when those categories would produce superfluous output under specified versions of F<perl>. For example, the C<redundant> category of warnings was introduced in Perl-5.22. This warning flags certain instances of superfluous arguments to C<printf> and C<sprintf>. But if your code was running warnings-free on earlier versions of F<perl> and you don't care about C<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 C<no if> construct assumes that a module or pragma has correctly implemented an C<unimport()> method -- but most modules and pragmata have not. That explains why the C<no if> construct is of limited applicability. =head1 BUGS The current implementation does not allow specification of the required version of the module. =head1 SEE ALSO L<Module::Requires> can be used to conditionally load one or modules, with constraints based on the version of the module. Unlike C<if> though, L<Module::Requires> is not a core module. L<Module::Load::Conditional> 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 L<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. =head1 AUTHOR Ilya Zakharevich L<mailto:ilyaz@cpan.org>. =head1 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. =cut