Now that we have seen how to use Perl 4 libraries, let's see what are the problems with them?

Although they are still in use, they belong to much earlier era. Before Perl 5 came out in 1995.

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All of nothing: The library (library.pl) we had in Perl 4 libraries had 3 functions in it and a global variable ($base). If we import the file as we have done in the perl4_app.pl file then we will have all the functions and variables of the library in our script.

The problem is that if you have more than one libraries used in the same script and some of the function names are declared in both. For example there is an add function of the library dealing with math and a totally different add function in a library dealing with inventory. If you write a script that requires both libraries then both would import the add function and they would override each other. More specifically the second one loaded (using require) would override the first one. Which means the load order of the requires will matter, which will probably drive crazy someone who is trying to understand why the script acts strangely.

This potential of collision just increases with the number of additional libraries used.

And perl does not even warn you about this, of course, because this is Perl 4 style coding without use warnings. (Although we could turn on warnings by adding -w to the sh-bang line:

#!/usr/bin/perl -w

Even more problematic is that now all the internal functions (validate_parameters in our case), and all the internal global variables ($base in our case) will be available and global in the script as well.

So in order to stay clear from interfering with the internals of the library, you need to actually know about the $base variable and you need to avoid it in your code.

It's even worse when the library is updated. Especially if the library writer is not aware of all the script using it then they can introduce a new function or a new variable that will start to collide with something in one of the scripts using that module.

Early Conclusion

While Perl libraries were useful, and they are still much better than copy-paste, their time is over. So we'll learn how to convert them to something more modern.

Prefix everything

One of the solutions to the problem of collision is to use prefixes. Prefix every function and every global variable name in the library with some specific word unique to that library. For example a library with functions related to math could be prefixed with _math_ or calc_ as in the example.

calc_perfix_lib.pl:

$calc_base = 10;

sub calc_add {
    calc_validate_parameters(@_);

    my $total = 0;
    $total += $_ for (@_);
    return $total;
}

sub calc_multiply {
}

sub calc_validate_parameters {
    die 'Not all of them are numbers'
        if  grep {/\D/} @_;
    return 1;
}

1;

This reduces the potential for collision in the scripts using this module, but the inconvenience of this solution is that now we need to use these prefixes everywhere, including inside the library. That requires more typing and code which is probably less readable.

Besides, Perl 5 has a perfectly nice solution for this using namespaces (usually also referred to as modules). We are going to talk about those in the next episode.