Design pattern factory : use lib issue on lib directory

CraigWalker has asked for the wisdom of the Perl Monks concerning the following question:

Hello Monks

I'm doing some preliminary tests on factory design pattern with PERL and I've found the following issue. If I implement my factory like this, I have no problem.

package GreetFactory;
use strict; use warnings;
use lib './Greet';

sub instantiate {
    my $class          = shift;
    my $requested_type = shift;
    my $location       = "Greet/$requested_type.pm";
    $class          = "Greet::$requested_type";
    require $location;
    return $class->new(@_);
}
1;
[download]

But if I implement my factory like that, then a load error is displayed (Can't locate object method "new" via package "lib::Repeat" (perhaps you forgot to load "lib::Repeat"?) at GreetFactory.pm line 13).

package GreetFactory;
use strict; use warnings;
use lib './lib';

sub instantiate {
    my $class          = shift;
    my $requested_type = shift;
    my $location       = "lib/$requested_type.pm";
    $class          = "lib::$requested_type";
    require $location;
    return $class->new(@_);
}
1;
[download]

Of course, with the 1st test, Repeat.pm is located into a 'Greet' subdirectory, and I rename it 'lib' before doing the 2nd test. Everything happens as if 'lib::' is badly interpreted because 'lib' may be a PERL reserved word ?

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Re: Design pattern factory : use lib issue on lib directory by Eily (Monsignor) on Apr 01, 2016 at 16:32 UTC
Renaming the directory isn't enough, you also have to change the package declaration at the top of Repeat.pm.	[reply]
Re^2: Design pattern factory : use lib issue on lib directory by CraigWalker (Initiate) on Apr 01, 2016 at 16:43 UTC
Hello, You are fully right. I've just checked the package was named package Greet.::Repeat; Sorry !	[reply]
Re: Design pattern factory : use lib issue on lib directory by Corion (Patriarch) on Apr 01, 2016 at 16:34 UTC
What is the namespace of the loaded file? In Perl , the filename and the namespace of the loaded class(es) can be different. If you didn't change the contents of the file `Greet/Repeat.pm` when you moved it to `lib/Repeat.pm`, it will still fill in a namespace `Great::Repeat`. Also see Module::Pluggable.	[reply] [d/l] [select]
Re: Design pattern factory : use lib issue on lib directory by haukex (Archbishop) on Apr 02, 2016 at 08:49 UTC
Hi CraigWalker, I'm not sure I understand your intent in the second example. Are you trying to relocate the class `Greet::Repeat` into the `lib/` subdirectory, or do you actually want to rename the `Greet::Repeat` class to `lib::Repeat`, as Eily and Corion have pointed out? If it's the former, and you want to follow the normal Perl convention of class and module naming, that means you'll have a file `lib/Greet/Repeat.pm` that contains the `Greet::Repeat` class. If so, then all you need to do to make your first piece of example code work is change the `use lib` statement to `use lib './lib';`. lib will prepend that path to `@INC`, so when you tell require to load the file `Greet/Repeat.pm`, it will look for `./lib/Greet/Repeat.pm` first. Also, the `use lib './Greet';` you've got in the first code example actually doesn't help you if the file is named `Greet/Repeat.pm` - require will first look for `./Greet/Greet/Repeat.pm`, which it doesn't find, then it looks through the rest of `@INC`. The first piece of example code works because the current directory `.` is in `@INC` by default. I think this can vary, but in my case `.` is last in `@INC`, so that perl actually looks through all the other `@INC` directories for a file named `Greet/Repeat.pm` first, which might not be what you want either. And AFAIK there are cases where the current directory isn't even in `@INC`, so that the script would fail there. Similarly, if your intent really was to rename `Greet::Repeat` to `lib::Repeat`, then the `use lib './lib';` in your second code example is also not necessary, since you've already added that path to the filename in `my $location = "lib/$requested_type.pm";`. In any case, I recommend that you don't rely on the current directory `.` as much. It's acceptable for initial testing, but in production I'd strongly recommend you use absolute paths for lib, or you use a different way of setting up `@INC` properly. Sometimes FindBin can help you (but it's not perfect, so be careful and test it first). (For example, if I've got a script I call from the command line and it needs a module that I know will always be in the same directory as the script, I'll sometimes write `use FindBin; use lib $FindBin::Bin; use ModuleName;`.) Hope this helps, -- Hauke D	[reply] [d/l] [select]