How can I conditionally use a module in Perl?

Question

I want to do something like this in Perl:

$Module1="ReportHashFile1"; # ReportHashFile1.pm
$Module2="ReportHashFile2"; # ReportHashFile2.pm

if(Condition1)
{
  use $Module1;
}
elsif(Condition2)
{
  use $Module2;
}

ReportHashFile*.pm contains a package ReportHashFile* .

Also how to reference an array inside module based on dynamic module name?

@Array= @$Module1::Array_inside_module;

Is there anyway I can achieve this. Some sort of compiler directive?

Answer 1

People have already told you how you can load the module with Perl primitives. There's also Module::Load::Conditional.

If you're looking to access an array of the same name no matter which module you loaded, consider making a method for that so you can skip the symbolic reference stuff. Give each module a method of the same name:

  package ReportHashFileFoo;
  our @some_package_variable;
  sub get_array { \@some_package_variable }

Then, when you load that module:

  if( ... some condition ... ) {
       eval "use $module" or croak ...;
       my $array_ref = $module->get_array;
       }

I don't know what you're really doing (XY Problem), but there's probably a better design. When things seem tricky like this, it's usually because you're overlooking a better way to to it.

Answer 2

You might find the if module useful for this.

Otherwise the basic idea is to use require, which happens at run-time, instead of use, which happens at compile-time. Note that '

BEGIN {
    my $module = $condition ? $Module1 : $Module2;
    my $file = $module;
    $file =~ s[::][/]g;
    $file .= '.pm';
    require $file;
    $module->import;
}

As for addressing globals, it might be easier if you just exported the variable or a function returning it to the caller, which you could use by its unqualified name. Otherwise there's also the possibility of using a method and calling it as $Module->method_name.

Alternatively, you could use symbolic references as documented in perlref. However, that's usually quite a code smell.

my @array = do {
    no strict 'refs';
    @{ ${ "${Module}::Array_inside_module" } };
};

Answer 3

Unless the execution speed is important, you can use string eval:

if (Condition1) {
    eval "use $Module1"; die $@ if $@;
}
elsif (Condition2) {
    eval "use $Module2"; die $@ if $@;
}