Is there a standard way to code a module to hold global application parameters to be included in every other package? For instance: use Config;?
A simple package that only contains our variables? What about readonly variables?
There's already a standard Config module, so choose a different name.
Say you have MyConfig.pm with the following contents:
package MyConfig;
our $Foo = "bar";
our %Baz = (quux => "potrzebie");
1;
Then other modules might use it as in
#! /usr/bin/perl
use warnings;
use strict;
use MyConfig;
print "Foo = $MyConfig::Foo\n";
print $MyConfig::Baz{quux}, "\n";
If you don't want to fully qualify the names, then use the standard Exporter module instead.
Add three lines to MyConfig.pm:
package MyConfig;
require Exporter;
our #ISA = qw/ Exporter /;
our #EXPORT = qw/ $Foo %Baz /;
our $Foo = "bar";
our %Baz = (quux => "potrzebie");
1;
Now the full package name is no longer necessary:
#! /usr/bin/perl
use warnings;
use strict;
use MyConfig;
print "Foo = $Foo\n";
print $Baz{quux}, "\n";
You could add a read-only scalar to MyConfig.pm with
our $READONLY;
*READONLY = \42;
This is documented in perlmod.
After adding it to #MyConfig::EXPORT, you might try
$READONLY = 3;
in a different module, but you'll get
Modification of a read-only value attempted at ./program line 12.
As an alternative, you could declare in MyConfig.pm constants using the constant module and then export those.
Don't use global variables for configuration and don't sotre configuration as code. I have an entire chapter in Mastering Perl about this.
Instead, make a configuration class that any other package can use to access the configuration data. It will be much easier in the long run to provide an interface to something you may want to change later than deal with the nuttiness you lock yourself into by scattering variables names you have to support for the rest of your life.
A config interface also gives you the benefit of composing new answers to configuration questions by combining the right bits of actual configuration data. You hide all of that behind a method and the higher levels don't have to see how it's implemented. For instance,
print "Hello!" unless $config->be_silent;
The be_silent answer can be triggered for multiple reasons the higher-level code doesn't need to know about. It could be from a user switch, that the program detected it is not interactive, and so on. It can also be flipped by options such as a debugging switch, which overrides all other preferences. No matter what you decide, that code line doesn't change because that statement only cares about the answer, not how you got the answer.
Related
I am stuck while creating a perl Moose module.
I have a global pm module.
package XYZ;
require Exporter;
our #ISA = qw(Exporter); ## EDIT missed this line
our #EXPORT_OK = qw($VAR);
my $VAR1 = 1;
our $VAR = {'XYZ' => $VAR1};
1;
I want to get $VAR in a Moose module I'm creating
package THIS;
use Moose;
use YAML::XS;
sub get_all_blocks{
my ($self) = #_;
require $self->get_pkg(); # this returns the full path+name of the above package
# i cannot use use lib+use since the get_pkg starts complaining
our $VAR;
print YAML::XS::Dump($XYZ::VAR); # this works
print YAML::XS::Dump($VAR); # this does not work
# i cannot use the scope resolution since XYZ would keep changing.
}
1;
Can someone please help me with accessing variable?
EDIT: Missed one line in the package XYZ code.
I cannot touch the package XYZ since it is owned/used by someone else, I can just use it :(
Exporting variables may easily lead to trouble.
Why not
package XYZ;
use strict;
use warnings;
use Exporter qw(import);
our #EXPORT_OK = qw(get_var);
my $VAR = '...'; # no need for "our" now
sub get_var { return $VAR }
...
1;
and then
package THIS;
use warnings;
use strict;
use XYZ qw(get_var);
my $var = get_var();
...
1;
See Exporter.
As for what you tried to do, there are two direct problems
$VAR from XYZ is never imported into THIS. If you need symbols from other packages you need to import them.† Those packages have to make them available first, so you need to add it to #EXPORT_OK as well.
Like above but with $VAR instead of get_var()
package XYZ;
...
use Exporter qw(import);
our #EXPORT_OK = qw($VAR);
our $VAR = '...'; # need be "our" for this
with
package THIS;
...
use XYZ qw($VAR);
print "$VAR\n";
Now $VAR can be used directly, including being written to (unless declared constant); that can change its value under the feet of yet other code, which may never even know about any of it.
Another way is to use #EXPORT and then those symbols are introduced into every program that says use Package;. I strongly recommend to only use #EXPORT_OK, when callers need to explicitly list what they want. That also nicely documents what is being used.
Even once you add that, there is still a variable with the same name in THIS, which hides (masks, shadows) the $XYZ::VAR. So remove our $VAR in THIS. This is an excellent example of one problem with globals. Once they're introduced we have to be careful about them always and everywhere.
But there are far greater problems with sharing variables across modules.
It makes code components entangled and the code gets harder and harder to work with. It runs contrary to principles of well defined scopes and modular design, it enables action at a distance, etc. Perl provides many good tools for structuring code and we rarely need globals and shared variables. It is telling that the Exporter itself warns against that.
Note how now my $VAR in XYZ is not visible outside XYZ; there is no way for any code outside XYZ to know about it or to access it.‡ When it is our then any code in the interpreter can write it simply as $XYZ::VAR, and without even importing it; that's what we don't want.
Of course that there may be a need for or good use of exporting variables, what can occasionally be found in modules. That is an exception though, to be used sparingly and carefully.
† Unless they're declared as package globals under a lexical alias via our in their package, in which case they can be used anywhere as $TheirPackageName::varname.
‡ This complete privacy is courtesy of my.
You do not want our $VAR; in THIS's namespace. That creates a lexical reference to $THIS::VAR. Not what you want.
Instead, you need to use properly:
use XYZ qw($VAR);
However, XYZ doesn't have an import to run here, so you need to update that. There are two ways to fix XYZ to do this - one is to import import, e.g., use Exporter qw(import);, the other is to derive off Exporter, e.g., use parent qw(Exporter);. Both of these will get XYZ->import(...) to work properly.
Once XYZ is useing Exporter correctly, then the use XYZ qw($VAR); line will cause perl to implicitly load XYZ and call XYZ->import(qw($VAR)), which will import that variable into your namespace.
Now, having answered your question, I will join others in suggesting that exporting variables is a very bad code smell, and probably is not the best / cleanest way to do what you want.
I'm writing a basic program whose core logic is split across several project-specific modules for cleanliness (keeping subroutines organised by their purpose in the program's logic).
Suddenly had trouble exposing an option from the main package in one of the modules, and using the our statement appeared to have no effect.
For brevity, I'll copy+paste an isolated test case I wrote to examine this behaviour:
main.pl
#!/usr/bin/perl
use warnings;
use strict;
use File::Basename;
# The variable to be read by the module.
our $verbose = 1;
# Load Output.pm from directory
use lib dirname "$0";
use Output;
write_message "Hello, world\n";
Output.pm
package Output;
use warnings;
use strict;
use parent "Exporter";
our #EXPORT = qw(write_message);
# Should be imported?
our $verbose;
sub write_message {
print $_[0] unless !$verbose;
}
1;
Expected result: "Hello, world"
Actual result: Dead silence
It's quite possible that what I'm trying to achieve isn't even possible in Perl, as this isn't the intended use of modules (and heck, I understand why that'd be the case).
I'm still quite new to Perl and there are some things I'm struggling to wrap my head around. I've seen people recommend using the our declarator to expose a variable across packages, but I can't understand why this isn't working.
PS: If anybody knows a better approach to splitting an app's program-specific logic between modules, I'd appreciate some pointers too. :) But first and foremost, I'd prefer to learn why our-ing a variable isn't working.
An our statement just creates a package variable (whereas my creates a lexical variable). It has nothing to do with exporting
The best option is probably to declare the variable in the Output package and access it as $Output::verbose elsewhere. Like this
main.pl
#!/usr/bin/perl
use strict;
use warnings;
use File::Basename;
use lib dirname $0;
use Output;
$Output::verbose = 1;
write_message "Hello, world\n";
Output.pm
package Output;
use strict;
use warnings;
use Exporter 5.57 'import';
our #EXPORT = qw/ write_message /;
our $verbose;
sub write_message {
print $_[0] if $verbose;
}
1;
Note that I have also removed the incorrect quotes from around $0, and ever since version 5.57 of Exporter it has been possible (and preferable) to import it's import subroutine instead of subclassing it
our declares a package variable in the current package. The one in main.pl refers to $main::verbose; the one in Output.pm refers to $Output::verbose.
You can use the full name $main::verbose to access the variable from anywhere, but you can't really "export" it because exporting refers to making symbols accessible to users of your module. You're trying to do the opposite.
I am creating a Perl equivalent to my Python project.
Description:
I have a base module "base.py" that is used by all my scripts via "from base import *"
The base module has common subroutines/functions that can be executed inside the scripts
My attempt for Perl was placing inside each script "use base.pm". However the subroutines in Perl were not locally imported to the script so I needed to make a call to the "base" module each time I wanted to execute a subroutine. What is the Perl equivalent to Python's "from base import *"?
A few things:
The Local namespace is good to use for your local modules. Perl specifically reserves Local for this purpose. No official module will ever be in the Local namespace.
Perl is not Python. Perl will do things a bit differently. Sometimes there's an exact equivalent, sometimes not. This does not have an exact equivalent. Close, but not exact.
Method #1: Don't Export Any Functions
All functions you defined in your modules are available if you prefix the full namespace to it. This is the easiest way to define and use functions from your modules and is the least likely to cause problems. It's easy to see where a function came from, and you won't have a problem with two modules using the same function name.
I have a module called Local::Base that has a single function in it. My program can use this function, by simply referring it as Local::Base::foo:
My Program
use strict;
use warnings;
use feature qw(say);
use Local::Base;
my $foo_string = Local::Base::foo("string");
say "Foo: $foo_string";
Local/Base.pm
package Local::Base;
use strict;
use warnings;
sub foo {
my $string = shift;
return qq(I've foo'd "$string"!);
}
1;
Method #2: Use the Exporter Pragma to Specify What to Automatically Export
Perl has a special pragma called Exporter that allows me to specify which modules will automatically be imported into my program.
This is not like Python where I can specify any defined function. Instead, I have to list the ones I want to export. This has a disadvantage over Python's way: In Python, if I write a new function, it's automatically imported without me having to do anything. It also has a big advantage over Python's way: In Python, if I write a new function, it's automatically imported without me having to do anything whether I wanted it imported or not. You can imagine if I wrote a private function I didn't want people to use. In Python, it would automatically be available. In Perl, it wouldn't be unless I specified it:
My Program
use strict;
use warnings;
use feature qw(say);
use Local::Base;
my $foo_string = foo("string");
say "Foo: $foo_string";
Local/Base.pm
package Local::Base;
use strict;
use warnings;
use Exporter 'import';
our #EXPORT = qw(foo);
sub foo {
my $string = shift;
return qq(I've foo'd "$string"!);
}
1;
Now, whenever I use Local::Base, the foo function is automatically imported.
Notice that I list all functions I want to export in the #EXPORT array. (Also note I declare that array with an our instead of a my. That our means the #EXPORT is a PACKAGE variable.). Also notice the qw(...) syntax. This is quote word. All words are separate elements of an array. You don't use commas:
my #array = ("one", "two", "three");
my #array = qw(one two three);
Both of these are equivalent. You also may see it this way:
my #array = qw/one two three/;
I like the parentheses, but the forward slashes tend to be the standard.
Method #3: Be Nice When Exporting
It is not recommended you use automatic exporting any more. Older modules like File::Copy still do it, but newer modules make you import your stuff. This also uses the Exporter pragma, but I specify #EXPORT_OK instead of just #EXPORT.
Now, when I specify I want to use Local::Base, I have to specify the functions I want to import into my program:
My Program
use strict;
use warnings;
use feature qw(say);
use Local::Base qw(foo);
my $foo_string = foo("string");
say "Foo: $foo_string";
Local/Base.pm
package Local::Base;
use strict;
use warnings;
use Exporter 'import';
our #EXPORT_OK; = qw(foo);
sub foo {
my $string = shift;
return qq(I've foo'd "$string"!);
}
1;
This forces the user to document the functions in your module they want to use. This way, they know where imported functions came from. It enforces good programming practice. Plus, if you use multiple modules and they have similar function names, you can make sure you use the one from the module you want. (Remember, you can still specify the package name prefixed before the function if you want to use the other one).
What if I am a Python programmer and I don't care about good programming practices? (Wait, that didn't come out quite right...) You can still (sort of) do it the Python way by specifying a Regular Expression (after all this is Perl):
use Local::Base '/.+/';
This will export all modules listed in both #EXPORT and #EXPORT_OK that match this regular expression. Since this matches everything, it will import everything you listed in #EXPORT and #EXPORT_OK. It won't import all functions. It will only import the functions in the #EXPORT and #EXPORT_OK arrays. Of course, you can specify any regular expressions and even ant-regular expressions. This will export all exportable functions except those with bar in the name:
use Local::Base '!/bar/';
Take a look at the Exporter pragma, and see what other goodies it has. For example, you can group functions into tags. That way, users can specify a particular set of functions with just a tag. See Math::Trig for a good example.
Hope this helps.
Sorry for the long answer, but I'm married with kids. Like, I'm doing anything else on New Years Eve.
You generally specify which functions to import just as a list of names:
use List::Util 'max', 'min';
Most modules that export things will follow these semantics:
use MyBase; # imports default exports (if any)
use MyBase 'baz'; # imports only baz
use MyBase (); # import nothing
Inside the module, an import class method is called that can choose what to export, usually having a default list but using the list passed if there is one. The Exporter module exists to help you do this:
package MyBase;
use Exporter 'import';
our #EXPORT = ( 'foo', 'bar' );
our #EXPORT_OK = ( 'baz', 'quux' );
There is also a facility for grouping exports by tag and allowing importing a whole group easily, see the Exporter docs. Variables can also be exported, not just subs.
With that background, to finally answer your question:
For modules that use Exporter (not all do), you can specify imports with a regular expression enclosed in //:
use List::Util '/./';
First of all base.pm is a very bad name as it is a core module. Second, exportable functions must be declared as such (usually), see Exporter for examples.
I have a question relating to Perl and scoping. I have a common file with lots of various variables. I require the common file in my main script, but I cannot access the variables; they seem to be outside of its scope. I assumed that an our declaration would overcome that problem, but it doesn't seem to work.
Script 1: common.pl
#!/usr/bin/perl
our $var1 = "something";
our $var2 = "somethingelse";
Script 2: ftp.pl
#!/usr/bin/perl
use strict;
use warnings;
require('common.pl');
print $var1;
I get the error: Global symbol "$var1" requires explicit package name
There's no require statement in your second example, but it wouldn't work anyway. What our does is declare a lexically-scoped package variable. Since you have no package statement, it uses the default package main. So your first script sets up the variable $main::var1, but this will only be available within that file's scope.
A better way to provide common variables for other scripts is to use Exporter. You can define package symbols in one place and Exporter will take care of copying them to the requesting script or class's namespace when needed.
I would put the config in a module instead.
File: MyConfig.pm
package MyConfig;
require Exporter;
use strict;
our #ISA = qw(Exporter);
our #EXPORT = qw( getconfig );
my %confighash = (
thisone => 'one',
thatone => 2,
somthingelse => 'froboz',
);
sub getconfig {
return %confighash;
}
1;
Example usage:
#!/usr/bin/perl
use strict;
use warnings;
use MyConfig;
my %config = getconfig();
print $config{ somthingelse };
This should print froboz
It looks like you need a proper configuration file there. I'd go for a non-code configuration file that you can read when you need to setup things. There are modules on CPAN to handle just about any configuration format you can imagine.
If you want to do it the way you have it, get rid of our and declare them with use vars. Don't let the PBP police scare you off that. :) You only really need our to limit a scope of a package variable, and that's exactly the opposite of what you are trying to do.
our() does something a little different than you think. Its sole purpose is to work with strict in requiring you to declare package variables that you are going to use (unless they are fully-qualified or imported). Like strict, its effect is lexically-scoped. To use it to allow accessing a global $main:var1 from multiple files (which are separate scopes) as just $var1, you need to say our $var1 in each file.
Alternatively, you would change your required file to be a module with its own package that exports the variables to any package that uses it.
Try this. I am new at Perl but this is how I got it to work on a script I made
#!/usr/bin/perl
$var1 = "something";
$var2 = "somethingelse";
Script 2: ftp.pl
#!/usr/bin/perl
use strict;
use warnings;
our $var1;
our $var2;
require('common.pl');
print $var1;
I'd like to create several modules that will be used in nearly all scripts and modules in my project. These could be used in each of my scripts like so:
#!/usr/bin/perl
use Foo::Bar;
use Foo::Baz;
use Foo::Qux;
use Foo::Quux;
# Potentially many more.
Is it possible to move all these use statements to a new module Foo::Corge and then only have to use Foo::Corge in each of my scripts and modules?
Yes, it is possible, but no, you shouldn't do it.
I just spent two weeks to get rid of a module that did nothing but use other modules. I guess this module started out simple and innocent. But over the years it grew into a huge beast with lots and lots of use-statements, most of which weren't needed for any given run of our webapp. Finally, it took some 20 seconds just to 'use' that module. And it supported lazy copy-and-paste module creation.
So again: you may regret that step in a couple of months or years. And what do you get on the plus side? You saved typing a couple of lines in a couple of modules. Big deal.
Something like this should work:
http://mail.pm.org/pipermail/chicago-talk/2008-March/004829.html
Basically, create your package with lots of modules:
package Lots::Of::Modules;
use strict; # strictly optional, really
# These are the modules we want everywhere we say "use Lots::Of::Modules".
# Any exports are re-imported to the module that says "use Lots::Of::Modules"
use Carp qw/confess cluck/;
use Path::Class qw/file dir/;
...
sub import {
my $caller = caller;
my $class = shift;
no strict;
*{ $caller. '::'. $_ } = \*{ $class. '::'. $_ }
for grep { !/(?:BEGIN|import)/ } keys %{ $class. '::' };
}
Then use Lots::Of::Modules elsewhere;
use Lots::Of::Modules;
confess 'OH NOES';
Yes.
In Foo/Corge.pm
use Foo::Bar;
use Foo::Baz;
use Foo::Qux;
use Foo::Quux;
1; # Be successful
All that is left is to get the directory containing sub-directory Foo added to your library path (#INC). Alternatively, create Foo.pm and have it use the other modules. They would be in a Foo sub -directory beside Foo.pm.
If you think about it, all the complex Perl modules that use other modules do this all the time. They are not necessarily in the same top-level package (Foo in this example), but they are used just as necessarily.
While you could use Carp, and Path::Class and confess, and so on (as jrockway suggests), that seems like overkill from where I'm sitting.
[EDIT: My earlier solution involving use Lots::Of::Modules; had a subtle bug -- see bottom. The fix makes things a bit uglier, but still workable.]
[EDIT #2: Added BEGIN { ... } around the code to ensure that any functions defined are available at compile time. Thanks to jrockway for pointing this out.]
The following code will do exactly what jrockway's code does, only simpler and clearer:
In Lots/Of/Modules.inc:
use Carp qw/confess cluck/;
use Path::Class qw/file dir/;
0; # Flag an error if called with "use" or "require" instead of "do"
To import those 4 functions:
BEGIN { defined( do 'Lots/Of/Modules.inc' ) or die; }
Because we don't have a package Lots::Of::Modules; statement at the start of this file, the use statements will export into the caller's package.
We must use do instead of use or require, since the latter will only load the file once (causing failure if use Lots::Of::Modules; is called more than once, e.g. in separate modules used by the main program). The more primitive do does not throw an exception if it fails to find the file named by its argument in #INC, hence the need for checking the result with defined.
Another option would be for Foo::Corge to just re-export any items of interest normally:
package Foo::Corge;
use base 'Exporter';
BEGIN {
our #EXPORT_OK = qw( bar baz qux quux );
use Foo::Bar qw( bar );
use Foo::Baz qw( baz );
use Foo::Qux qw( qux );
use Foo::Quux qw( quux );
}
1;
(The 'use' statements can probably go outside the BEGIN, but that's where they were in the code I checked to verify that this worked the way I thought it did. That code actually evals the uses, so it has a reason for them to be inside BEGIN which likely doesn't apply in your case.)
using #EXPORT instead #EXPORT_OK , is more simple
Library is :
package mycommon;
use strict;
use warnings;
use base 'Exporter';
our #EXPORT = qw(test);
sub test {
print "this is a test";
}
1;
use it:
#!/usr/bin/perl
use strict;
use warnings;
use mycommon;
common::test()