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Member "install-tl-20200916/tlpkg/tlperl/lib/if.pm" (10 Mar 2019, 3611 Bytes) of package /windows/misc/install-tl.zip:


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    1 package if;
    2 
    3 $VERSION = '0.0608';
    4 
    5 sub work {
    6   my $method = shift() ? 'import' : 'unimport';
    7   unless (@_ >= 2) {
    8     my $type = ($method eq 'import') ? 'use' : 'no';
    9     die "Too few arguments to '$type if' (some code returning an empty list in list context?)"
   10   }
   11   return unless shift;      # CONDITION
   12 
   13   my $p = $_[0];        # PACKAGE
   14   (my $file = "$p.pm") =~ s!::!/!g;
   15   require $file;        # Works even if $_[0] is a keyword (like open)
   16   my $m = $p->can($method);
   17   goto &$m if $m;
   18 }
   19 
   20 sub import   { shift; unshift @_, 1; goto &work }
   21 sub unimport { shift; unshift @_, 0; goto &work }
   22 
   23 1;
   24 __END__
   25 
   26 =head1 NAME
   27 
   28 if - C<use> a Perl module if a condition holds
   29 
   30 =head1 SYNOPSIS
   31 
   32     use if CONDITION, "MODULE", ARGUMENTS;
   33     no  if CONDITION, "MODULE", ARGUMENTS;
   34 
   35 =head1 DESCRIPTION
   36 
   37 =head2 C<use if>
   38 
   39 The C<if> module is used to conditionally load another module.  The construct:
   40 
   41     use if CONDITION, "MODULE", ARGUMENTS;
   42 
   43 ... will load C<MODULE> only if C<CONDITION> evaluates to true; it has no
   44 effect if C<CONDITION> evaluates to false.  (The module name, assuming it
   45 contains at least one C<::>, must be quoted when C<'use strict "subs";'> is in
   46 effect.)  If the CONDITION does evaluate to true, then the above line has the
   47 same effect as:
   48 
   49     use MODULE ARGUMENTS;
   50 
   51 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:
   52 
   53     use if defined &DynaLoader::boot_DynaLoader,
   54         "Unicode::Normalize" => qw(getCombinClass NFD);
   55 
   56 Suppose you wanted C<ARGUMENTS> to be an empty list, I<i.e.>, to have the
   57 effect of:
   58 
   59     use MODULE ();
   60 
   61 You can't do this with the C<if> pragma; however, you can achieve
   62 exactly this effect, at compile time, with:
   63 
   64     BEGIN { require MODULE if CONDITION }
   65 
   66 =head2 C<no if>
   67 
   68 The C<no if> construct is mainly used to deactivate categories of warnings
   69 when those categories would produce superfluous output under specified
   70 versions of F<perl>.
   71 
   72 For example, the C<redundant> category of warnings was introduced in
   73 Perl-5.22.  This warning flags certain instances of superfluous arguments to
   74 C<printf> and C<sprintf>.  But if your code was running warnings-free on
   75 earlier versions of F<perl> and you don't care about C<redundant> warnings in
   76 more recent versions, you can call:
   77 
   78     use warnings;
   79     no if $] >= 5.022, q|warnings|, qw(redundant);
   80 
   81     my $test    = { fmt  => "%s", args => [ qw( x y ) ] };
   82     my $result  = sprintf $test->{fmt}, @{$test->{args}};
   83 
   84 The C<no if> construct assumes that a module or pragma has correctly
   85 implemented an C<unimport()> method -- but most modules and pragmata have not.
   86 That explains why the C<no if> construct is of limited applicability.
   87 
   88 =head1 BUGS
   89 
   90 The current implementation does not allow specification of the required
   91 version of the module.
   92 
   93 =head1 SEE ALSO
   94 
   95 L<Module::Requires> can be used to conditionally load one or modules,
   96 with constraints based on the version of the module.
   97 Unlike C<if> though, L<Module::Requires> is not a core module.
   98 
   99 L<Module::Load::Conditional> provides a number of functions you can use to
  100 query what modules are available, and then load one or more of them at runtime.
  101 
  102 The L<provide> module from CPAN can be used to select one of several possible
  103 modules to load based on the version of Perl that is running.
  104 
  105 =head1 AUTHOR
  106 
  107 Ilya Zakharevich L<mailto:ilyaz@cpan.org>.
  108 
  109 =head1 COPYRIGHT AND LICENCE
  110 
  111 This software is copyright (c) 2002 by Ilya Zakharevich.
  112 
  113 This is free software; you can redistribute it and/or modify it under
  114 the same terms as the Perl 5 programming language system itself.
  115 
  116 =cut