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.
Either break up the use
into its separate require
and import
components, or else employ the use
autouse
pragma.
Programs that check their arguments and abort with a usage message on error have no reason to load modules they'll 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: $0 num1 num2\n"; } } use Some::Module; use More::Modules;
A related situation arises in programs that don't always use the same set of modules every time they're run. For example, the factors program from Chapter 2, Numbers, needs the infinite precision arithmetic library only when the -b command-line flag is supplied. A use
statement would be pointless within a conditional because it's evaluated at compile time, long before the if
can be checked. So we'll use a require
instead:
if ($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 run time means that the rest of your program will not be 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 will not be 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't use indirect objects on anything but a plain scalar variable, or 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.
A convenient alternative is the use
autouse
pragma. New as of Perl 5.004, 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 autouse
d them but not when we use
d them or import
ed 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 will be 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.
Recipe 12.2; the discussion on the import
method in the documentation for the standard Exporter module, also found in Chapter 7 of Programming Perl; the documentation for the standard use
autouse
pragma
Copyright © 2001 O'Reilly & Associates. All rights reserved.