Recipe 12.3 Delaying use Until Runtime

12.3.1 Problem

You have a module that you don't need to load each time the program runs, or whose inclusion you wish to delay until after the program starts up.

12.3.2 Solution

Either break up the use into its separate require and import components, or else employ the use autouse pragma.

12.3.3 Discussion

Programs that check their arguments and abort with a usage message on error have no reason to load modules they never use. This delays the inevitable and annoys users. But those use statements happen during compilation, not execution, as explained in the Introduction.

Here, an effective strategy is to place argument checking in a BEGIN block before loading the modules. The following is the start of a program that checks to make sure it was called with exactly two arguments, which must be whole numbers, before going on to load the modules it will need:

BEGIN {
    unless (@ARGV =  = 2 && (2 =  = grep {/^\d+$/} @ARGV)) {
        die "usage: $0 num1 num2\n";
    }
}
use Some::Module;
use More::Modules;

A related situation arises in programs that don't always use the same set of modules every time they're run. For example, the factors program from Chapter 2 needs the infinite precision arithmetic library only when the -b command-line flag is supplied. A use statement would be pointless within a conditional because it's evaluated at compile time, long before the if can be checked. So we use a require instead:

if ($opt_b) {
    require Math::BigInt;
}

Because Math::BigInt is an object-oriented module instead of a traditional one, no import was needed. If you have an import list, specify it with a qw( ) construct as you would with use. For example, rather than this:

use Fcntl qw(O_EXCL O_CREAT O_RDWR);

you might say this instead:

require Fcntl;
Fcntl->import(qw(O_EXCL O_CREAT O_RDWR));

Delaying the import until runtime means that the rest of your program is not subject to any imported semantic changes that the compiler would have seen if you'd used a use. In particular, subroutine prototypes and the overriding of built-in functions are not seen in time.

You might want to encapsulate this delayed loading in a subroutine. The following deceptively simple approach does not work:

sub load_module {
    require $_[0];  #WRONG
    import  $_[0];  #WRONG
}

It fails for subtle reasons. Imagine calling require with an argument of "Math::BigFloat". If that's a bareword, the double colon is converted into your operating system's path separator and a trailing .pm is added. But as a simple variable, it's a literal filename. Worse, Perl doesn't have a built-in import function. Instead, there's a class method named import that we're using the dubious indirect object syntax on. As with indirect filehandles, you can use indirect objects only on a plain scalar variable, a bareword, or a block returning the object, not an expression or one element from an array or hash.

A better implementation might look more like:

load_module("Fcntl", qw(O_EXCL O_CREAT O_RDWR));

sub load_module {
    eval "require $_[0]";
    die if $@;
    $_[0]->import(@_[1 .. $#_]);
}

But this still isn't perfectly correct in the general case. It really shouldn't import those symbols into its own package. It should put them into its caller's package. We could account for this, but the whole procedure is getting increasingly messy.

Occasionally, the condition can be reasonably evaluated before runtime, perhaps because it uses only built-in, predefined variables, or because you've arranged to initialize the variables used in the conditional expression at compile time with a BEGIN block. If so, the if pragma comes in handy. The syntax is:

use CONDITION, MODULE;
use CONDITION, MODULE => ARGUMENTS;

As in:

use if $^O =~ /bsd/i, BSD::Resource;
use if $] >= 5.006_01, File::Temp => qw/tempfile tempdir/;

A convenient alternative is the use autouse pragma. This directive can save time on infrequently loaded functions by delaying their loading until they're actually used:

use autouse Fcntl => qw( O_EXCL( ) O_CREAT( ) O_RDWR( ) );

We put parentheses after O_EXCL, O_CREAT, and O_RDWR when we autoused them but not when we used them or imported them. The autouse pragma doesn't just take function names; it can also take a prototype for the function. The Fcntl constants are prototyped to take no arguments, so we can use them as barewords in our program without use strict kvetching.

Remember, too, that use strict's checks take place at compile time. If we use Fcntl, the prototypes in the Fcntl module are compiled and we can use the constants without parentheses. If we require or wrap the use in an eval, as we did earlier, we prevent the compiler from reading the prototypes, so we can't use the Fcntl constants without parentheses.

Read the autouse pragma's online documentation to learn its various caveats and provisos.

12.3.4 See Also

Recipe 12.2; the discussion on the import method in the documentation for the standard Exporter module, also found in Chapter 32 of Programming Perl; the documentation for the standard use autouse pragma