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package if;
use strict;
our $VERSION = '0.0609';
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
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