if(3p) Perl Programmers Reference Guide if(3p) #
if(3p) Perl Programmers Reference Guide if(3p)
NNAAMMEE #
if - "use" a Perl module if a condition holds
SSYYNNOOPPSSIISS #
use if CONDITION, "MODULE", ARGUMENTS;
no if CONDITION, "MODULE", ARGUMENTS;
DDEESSCCRRIIPPTTIIOONN #
“"uussee iiff"” 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 _U_n_i_c_o_d_e_:_:_U_C_D module's _c_h_a_r_i_n_f_o function will use two
functions from _U_n_i_c_o_d_e_:_:_N_o_r_m_a_l_i_z_e 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 }
“"nnoo iiff"” The “no if” construct is mainly used to deactivate categories of warnings when those categories would produce superfluous output under specified versions of _p_e_r_l.
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 _p_e_r_l 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.
BBUUGGSS #
The current implementation does not allow specification of the required
version of the module.
SSEEEE AALLSSOO #
Module::Requires can be used to conditionally load one or more modules,
with constraints based on the version of the module. Unlike "if" though,
Module::Requires is not a core module.
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 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.
AAUUTTHHOORR #
Ilya Zakharevich <mailto:ilyaz@cpan.org>.
CCOOPPYYRRIIGGHHTT AANNDD LLIICCEENNCCEE #
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 v5.36.3 2023-02-15 if(3p)