117 lines
3.5 KiB
Perl
117 lines
3.5 KiB
Perl
package if;
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use strict;
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our $VERSION = '0.0610';
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sub work {
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my $method = shift() ? 'import' : 'unimport';
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unless (@_ >= 2) {
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my $type = ($method eq 'import') ? 'use' : 'no';
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die "Too few arguments to '$type if' (some code returning an empty list in list context?)"
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}
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return unless shift; # CONDITION
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my $p = $_[0]; # PACKAGE
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(my $file = "$p.pm") =~ s!::!/!g;
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require $file; # Works even if $_[0] is a keyword (like open)
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my $m = $p->can($method);
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goto &$m if $m;
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}
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sub import { shift; unshift @_, 1; goto &work }
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sub unimport { shift; unshift @_, 0; goto &work }
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1;
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__END__
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=head1 NAME
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if - C<use> a Perl module if a condition holds
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=head1 SYNOPSIS
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use if CONDITION, "MODULE", ARGUMENTS;
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no if CONDITION, "MODULE", ARGUMENTS;
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=head1 DESCRIPTION
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=head2 C<use if>
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The C<if> module is used to conditionally load another module. The construct:
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use if CONDITION, "MODULE", ARGUMENTS;
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... will load C<MODULE> only if C<CONDITION> evaluates to true; it has no
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effect if C<CONDITION> evaluates to false. (The module name, assuming it
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contains at least one C<::>, must be quoted when C<'use strict "subs";'> is in
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effect.) If the CONDITION does evaluate to true, then the above line has the
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same effect as:
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use MODULE ARGUMENTS;
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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:
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use if defined &DynaLoader::boot_DynaLoader,
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"Unicode::Normalize" => qw(getCombinClass NFD);
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Suppose you wanted C<ARGUMENTS> to be an empty list, I<i.e.>, to have the
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effect of:
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use MODULE ();
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You can't do this with the C<if> pragma; however, you can achieve
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exactly this effect, at compile time, with:
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BEGIN { require MODULE if CONDITION }
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=head2 C<no if>
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The C<no if> construct is mainly used to deactivate categories of warnings
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when those categories would produce superfluous output under specified
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versions of F<perl>.
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For example, the C<redundant> category of warnings was introduced in
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Perl-5.22. This warning flags certain instances of superfluous arguments to
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C<printf> and C<sprintf>. But if your code was running warnings-free on
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earlier versions of F<perl> and you don't care about C<redundant> warnings in
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more recent versions, you can call:
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use warnings;
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no if $] >= 5.022, q|warnings|, qw(redundant);
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my $test = { fmt => "%s", args => [ qw( x y ) ] };
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my $result = sprintf $test->{fmt}, @{$test->{args}};
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The C<no if> construct assumes that a module or pragma has correctly
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implemented an C<unimport()> method -- but most modules and pragmata have not.
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That explains why the C<no if> construct is of limited applicability.
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=head1 BUGS
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The current implementation does not allow specification of the required
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version of the module.
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=head1 SEE ALSO
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L<Module::Requires> can be used to conditionally load one or more modules,
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with constraints based on the version of the module.
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Unlike C<if> though, L<Module::Requires> is not a core module.
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L<Module::Load::Conditional> provides a number of functions you can use to
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query what modules are available, and then load one or more of them at runtime.
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The L<provide> module from CPAN can be used to select one of several possible
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modules to load based on the version of Perl that is running.
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=head1 AUTHOR
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Ilya Zakharevich L<mailto:ilyaz@cpan.org>.
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=head1 COPYRIGHT AND LICENCE
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This software is copyright (c) 2002 by Ilya Zakharevich.
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This is free software; you can redistribute it and/or modify it under
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the same terms as the Perl 5 programming language system itself.
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=cut
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