Question

I am experiencing a problem with using a constant defined in a configuration file.
This is my package:

package myPackage;
require "APIconfig.pl";
APIconfig::import(APIconfig);

use constant SERVICE_URL => APIconfig::SERVICE_URL();

The configuration looks like this:

package APIconfig;
use constant SERVICE_URL => 'http://api.example.org/blah';
1;

When running this code, I get the following error:

Undefined subroutine &APIconfig::SERVICE_URL called at API.pl line 4.

I cannot use ‘use’ instead of ‘require’ because this expects the configuration file to be named .pm, and it’s called .pl on a lot of servers on our network.
How can I use the package without renaming the file?

Answer 1

There are two differences between ‘use’ and ‘require’. One of them affects your current problem, the other doesn’t. Unfortunately you are working around one that has no effect.

The differences are:

1/ ‘use’ calls the import() function, ‘require’ doesn’t.

2/ ‘use’ happens at compile time, ‘require’ happens at runtime.

You’re working around the fact that ‘require’ doesn’t call import() by calling it explicitly. This has no effect as your module doesn’t export any symbols and doesn’t have an import() subroutine.

You’re not working around the fact that ‘use’ statements are executed at runtime. The problem is that “use constant SERVICE_URL => APIconfig::SERVICE_URL();” is executed at compile time and your ‘require’ hasn’t run by then so myPackage knows nothing about APIconfig.

The (nasty, hacky) solution is to put the ‘require’ statement into a BEGIN block – to force it to be executed at compile time. You’ll also want to remove the call to import() as that gives a runtime error (due to the absence of the subroutine).

The test files that I used to work this out are as follows:

$ cat APIconfig.pl 
package APIconfig;
use constant SERVICE_URL => 'http://api.example.org/blah';
1;

$ cat api.pl 
#!/usr/bin/perl

package myPackage;
BEGIN {
  require "APIconfig.pl";
}
# APIconfig::import(APIconfig);

use constant SERVICE_URL => APIconfig::SERVICE_URL();

print SERVICE_URL, "\n";
$ ./api.pl 
http://api.example.org/blah

The real solution is to rewrite APIconfig as a real module. You hint that you know that, but that environmental issues prevent you taking this approach. I highly recommend trying to work around those issues and doing things correctly.