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Perl: conditional use and scope

March 2010 Greg Bacon
#Perl

A reader asks

If I conditionally load a perl module, do those module variables get passed to the whole perl script?

if ( some_test ) {
  use "perlmodule_001";
}
else {
  use "perlmodule_002";
}

Are the elements of either perl module available outside the if statement?

The main program from the question has a syntax error:

syntax error at prog0 line 2, near "use "perlmodule_001""

Perl’s documentation for use explains:

use Module
Imports some semantics into the current package from the named module, generally by aliasing certain subroutine or variable names into your package. It is exactly equivalent to

BEGIN { require Module; Module->import( LIST ); }

except that Module must be a bareword.

Note the bareword constraint at the end: the compiler doesn’t like the double quotes around the argument to use. Our friend was likely thinking of the older require operator that does accept strings and arbitrary expressions in general.

Say we have two modules with alternative definitions of $Foo and $Bar:

package Perlmodule_001;

use Exporter 'import';
our @EXPORT = qw/ $Foo $Bar /;
our $Foo = 'apple';
our $Bar = 'orange';

1;

and

package Perlmodule_002;

use Exporter 'import';
our @EXPORT = qw/ $Foo $Bar /;
our $Foo = 42;
our $Bar = 'w00t!';

1;

Note the use of Perlmodule_001, for example, rather than perlmodule_001: the perlmodlib documentation notes, “Perl informally reserves lowercase module names for pragma modules like integer and strict.”

Consider the following simple driver:

#! /usr/bin/perl

use warnings;
use strict;

if (@ARGV && $ARGV[0] eq "two") {
    use Perlmodule_002;
}
else {
    use Perlmodule_001;
}

sub maybeUndef {
    defined $_[0] ? $_[0] : "<undefined>";
    # got 5.10?
    # $_[0] // "<undefined>";
}

print "Foo = ", maybeUndef($Foo), "\n",
      "Bar = ", maybeUndef($Bar), "\n";

It uses maybeUndef to explicitly show when a value is undefined and also to silence potential undefined-value warnings.

The program seems to run as intended

$ ./prog1
Foo = apple
Bar = orange

but the output is the same even when an argument of two is supplied on the command line!

$ ./prog1 two
Foo = apple
Bar = orange

The good news is that the imported variables are in scope for the rest of the program, as indicated in the above documentation for use (with emphasis added):

Imports some semantics into the current package from the named module …

To understand why we never see Perlmodule_002’s $Foo and $Bar, note that use “is exactly equivalent to” require at BEGIN time, and the perlmod documentation explains exactly when that is (with added emphasis):

A BEGIN code block is executed as soon as possible, that is, the moment it is completely defined, even before the rest of the containing file (or string) is parsed.

So the compiler sees use Perlmodule_002 and processes it. Then it sees use Perlmodule_001 and processes it. When the compiler finishes digesting the rest of the code, it’s time for the execution phase, when the @ARGV check finally takes place. As written, Perlmodule_001 will always win!

Because ordinary modules affect the current package, use–ing an ordinary module inside a conditional block is entirely misleading. I was careful to qualify the previous statement for ordinary modules because the effects of some pragmatic modules (e.g., strict and integer—note the lowercase names!) are limited tightly to the enclosing block only.

The fix is to process @ARGV at BEGIN time and conditionalize the module imports with the equivalent require and import:

#! /usr/bin/perl

use warnings;
use strict;

BEGIN {
    if (@ARGV && $ARGV[0] eq "two") {
        require Perlmodule_002;
        Perlmodule_002->import;
    }
    else {
        require Perlmodule_001;
        Perlmodule_001->import;
    }
}

sub maybeUndef {
    defined $_[0] ? $_[0] : "<undefined>";
    # got 5.10?
    # $_[0] // "<undefined>";
}

print "Foo  = ", maybeUndef($Foo), "\n",
      "Bar  = ", maybeUndef($Bar), "\n";

An alternative is protecting use with eval as in

BEGIN {
    if (@ARGV && $ARGV[0] eq "two") {
        eval "use Perlmodule_002";
    }
    # ...

so a particular use runs only when control reaches its eval but is ignored otherwise. This is a safe, sensible use of eval.

Either way, the program now does what we expect!

$ ./prog2
Foo  = apple
Bar  = orange
$ ./prog2 two
Foo  = 42
Bar  = w00t!

You might wonder why the code has to be inside a BEGIN block after the use lines are conditionalized. If you have the strict pragma enabled—and you should!—it wants variables to be imported and declared before execution begins. Otherwise, compilation will fail because for all it knows, $Foo and $Bar in the main package were typos.

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