Short version
Is it possible to access variables from a module declared as our using unqualified names within the BEGIN code block, but using qualified names outside? In particular, can this be done without explicitely naming the package in the module file?
Example
Let demomod.pm be
use strict;
use warnings;
package demomod;
our $foo;
BEGIN { $foo = 42; }
1;
and demoscript.pl be
#!/usr/bin/perl -Tw
use strict;
use warnings;
BEGIN { #INC = ('.', #INC); }
use demomod;
print $demomod::foo."\n";
In this case, all names agree, and everything works as it should. Is there a way to omit the line package demomod; from the demomod.pm code and still let this work?
Motivation
The reason why I'm asking is because I encountered something along these lines during a recent upgrade of Foswiki. That software has a module Foswiki.pm which does not have a package line (EDIT: seems the package line only got lost in my local copy, for reasons unknown). It declares and initializes a variable $engine like in my example. There also is a CGI script called view which sets #INC and then does use Foswiki (); followed by $Foswiki::engine->run(). This last line always fails for me due to the variable not being initialized:
Can't call method "run" on an undefined value at …/view
In the BEGIN block of the module, $engine is set correctly but $Foswiki::engine apparently is not. So it looks like there were two variables here, one qualified and a different one unqualified.
All that code apparently works for others, and a previous version used to work for me as well, without a package line either. So while I try to understand how this broke, I also try to understand how this could work before, without that line in place. Is there some mechanism that would make this work?
If you have no package statement in your code then any package variables will be declared into the main package. So no, you cannot do what you describe.
If you look at line 2 of the Foswiki code that you linked, you will see that it does have a package statement.
Related
Consider the following script p.pl:
use strict;
use warnings;
use AA;
BB::bfunc();
where the file AA.pm is:
package AA;
use BB;
1;
and the file BB.pm is:
package BB;
sub bfunc {
print "Running bfunc..\n";
}
1;
Running p.pl gives output (with no warnings or errors):
Running bfunc..
Q: Why is it possible to call BB::bfunc() from p.pl even though there is no use BB; in p.pl? Isn't this odd behavior? Or are there situation where this could be useful?
(To me, it seems like this behavior only presents an information leak to another package and violates the data hiding principle.. Leading to programs that are difficult to maintain.. )
You're not polluting a namespace, because the function within BB isn't being 'imported' into your existing namespace.
They are separate, and may be referenced autonomously.
If you're making a module, then usually you'll define via Exporter two lists:
#EXPORT and #EXPORT_OK.
The former is the list of things that should be imported when you use the package. The latter is the things that you can explicity import via:
use MyPackage qw ( some_func );
You can also define package variables in your local namespace via our and reference them via $main.
our $fish = "haddock";
print $main::fish;
When you do this, you're explicitly referencing the main namespace. When you use a module, then you cause perl to go and look for it, and include it in your %INC. I then 'knows about' that namespace - because it must in order for the dependencies to resolve.
But this isn't namespace pollution, because it doesn't include anything in your namespace until your ask.
This might make a bit more sense if you have multiple packages within the same program:
use strict;
use warnings;
package CC;
our $package_var = "Blong";
sub do_something {
print $package_var,"\n";
}
package main;
use Data::Dumper;
our $package_var = "flonk";
print Dumper $package_var;
print Dumper $CC::package_var;
Each package is it's own namespace, but you can 'poke' things in another. perl will also let you do this with object - poking at the innards of instantiated objects or indeed "patch" them.
That's quite powerful, but I'd generally suggest Really Bad Style.
While it's good practice to use or require every dependency that you are planning to access (tried to avoid use here), you don't have to do that.
As long as you use full package names, that is fine. The important part is that Perl knows about the namespaces. If it does not, it will fail.
When you use something, that is equivalent to:
BEGIN {
require Foo::Bar;
Foo::Bar->import();
}
The require will take the Foo::Bar and convert it to a path according to the operating system's conventions. On Linux, it will try to find Foo/Bar.pm somewhere inside #INC. It will then load that file and make a note in %INC that it loaded the file.
Now Perl knows about that namespace. In case of the use it might import something into your own namespace. But it will always be available from everywhere after that as long as you use the full name. Just the same, stuff that you have in your main script.pl would be available inside of packages by saying main::frobnicate(). (Please don't do that!)
It's also not uncommon to bundle several namespaces/packages in one .pm module file. There are quite a few big names on CPAN that do it, like XML::Twig.
If you do that, and don't import anything, the only way to get to the stuff under the different namespaces is by using the full name.
As you can see, this is not polluting at all.
I am new to Perl and I have been following a book. All is well, except for whenever I try to initialize a variable as shown by the book, I am getting an error like below. Kindly tell me what should I do in order to avoid this error?
Code:
#!/usr/bin/perl -w
use 5.014;
use strict;
use utf8;
$radius = <STDIN>;
$circum;
if ($radius<0){
$circum = 0
} else{
$circum = 2*3.141*$radius;
}
print $circum."\n";
Errors:
Global symbol "$radius" requires explicit package name at ./example1 line 6.
Global symbol "$circum" requires explicit package name at ./example1 line 7.
Global symbol "$radius" requires explicit package name at ./example1 line 8.
Global symbol "$circum" requires explicit package name at ./example1 line 9.
Global symbol "$circum" requires explicit package name at ./example1 line 11.
Global symbol "$radius" requires explicit package name at ./example1 line 11.
Global symbol "$circum" requires explicit package name at ./example1 line 14.
Execution of ./example1 aborted due to compilation errors.
Also, I have read somewhere in the forums about 'our' and 'my' keywords. Using these seems to work. But, is it compulsory to use these keywords. If so, I think it is strange that the book did not include them.
The issue is the 'use strict' (which is actually a very good thing).
The result is that your variables need to be declared as follows:
my $radius = <STDIN>;
The 'my' keyword, there, makes the variable local to the current scope. And the 'use strict;' says you must declare all variables and specify their scope. The two typical ways you'd want to declare variables are:
my $localOnly;
our $shareableVariable;
Basically: use 'my' when you don't want anyone else to access the variable, and use 'our' when you want to allow external code to access or set the variable.
You've already got (and accepted) a answer to this question. But it might be worth raising another couple of points.
Firstly, if you don't understand a Perl error message, then it's often worth adding use diagnostics to your code. That will give you a more detailed explanation of the error. In this case, it would say:
(F) You've said "use strict" or "use strict vars", which indicates
that all variables must either be lexically scoped (using "my" or
"state"), declared beforehand using "our", or explicitly qualified to
say which package the global variable is in (using "::").
(Which, incidentally, shows the small omissions in the previous answer.)
Secondly, are you saying that your book recommends use strict but doesn't mention my? That sounds very strange. In any case, a Perl book that doesn't mention my is not a very good Perl book. Please tell us the title so that we can avoid it.
What I want to achieve:
###############CODE########
old_procedure(arg1, arg2);
#############CODE_END######
I have a huge code which has a old procedure in it. I want that the call to that old_procedure go to a call to a new procedure (new_procedure(arg1, arg2)) with the same arguments.
Now I know, the question seems pretty stupid but the trick is I am not allowed to change the code or the bad_function. So the only thing I can do it create a procedure externally which reads the code flow or something and then whenever it finds the bad_function, it replaces it with the new_function. They have a void type, so don't have to worry about the return values.
I am usng perl. If someone knows how to atleast start in this direction...please comment or answer. It would be nice if the new code can be done in perl or C, but other known languages are good too. C++, java.
EDIT: The code is written in shell script and perl. I cannot edit the code and I don't have location of the old_function, I mean I can find it...but its really tough. So I can use the package thing pointed out but if there is a way around it...so that I could parse the thread with that function and replace function calls. Please don't remove tags as I need suggestions from java, C++ experts also.
EDIT: #mirod
So I tried it out and your answer made a new subroutine and now there is no way of accessing the old one. I had created an variable which checks the value to decide which way to go( old_sub or new_sub)...is there a way to add the variable in the new code...which sends the control back to old_function if it is not set...
like:
use BadPackage; # sub is defined there
BEGIN
{ package BapPackage;
no warnings; # to avoid the "Subroutine bad_sub redefined" message
# check for the variable and send to old_sub if the var is not set
sub bad_sub
{ # good code
}
}
# Thanks #mirod
This is easier to do in Perl than in a lot of other languages, but that doesn't mean it's easy, and I don't know if it's what you want to hear. Here's a proof-of-concept:
Let's take some broken code:
# file name: Some/Package.pm
package Some::Package;
use base 'Exporter';
our #EXPORT = qw(forty_two nineteen);
sub forty_two { 19 }
sub nineteen { 19 }
1;
# file name: main.pl
use Some::Package;
print "forty-two plus nineteen is ", forty_two() + nineteen();
Running the program perl main.pl produces the output:
forty-two plus nineteen is 38
It is given that the files Some/Package.pm and main.pl are broken and immutable. How can we fix their behavior?
One way we can insert arbitrary code to a perl command is with the -M command-line switch. Let's make a repair module:
# file: MyRepairs.pm
CHECK {
no warnings 'redefine';
*forty_two = *Some::Package::forty_two = sub { 42 };
};
1;
Now running the program perl -MMyRepairs main.pl produces:
forty-two plus nineteen is 61
Our repair module uses a CHECK block to execute code in between the compile-time and run-time phase. We want our code to be the last code run at compile-time so it will overwrite some functions that have already been loaded. The -M command-line switch will run our code first, so the CHECK block delays execution of our repairs until all the other compile time code is run. See perlmod for more details.
This solution is fragile. It can't do much about modules loaded at run-time (with require ... or eval "use ..." (these are common) or subroutines defined in other CHECK blocks (these are rare).
If we assume the shell script that runs main.pl is also immutable (i.e., we're not allowed to change perl main.pl to perl -MMyRepairs main.pl), then we move up one level and pass the -MMyRepairs in the PERL5OPT environment variable:
PERL5OPT="-I/path/to/MyRepairs -MMyRepairs" bash the_immutable_script_that_calls_main_pl.sh
These are called automated refactoring tools and are common for other languages. For Perl though you may well be in a really bad way because parsing Perl to find all the references is going to be virtually impossible.
Where is the old procedure defined?
If it is defined in a package, you can switch to the package, after it has been used, and redefine the sub:
use BadPackage; # sub is defined there
BEGIN
{ package BapPackage;
no warnings; # to avoid the "Subroutine bad_sub redefined" message
sub bad_sub
{ # good code
}
}
If the code is in the same package but in a different file (loaded through a require), you can do the same thing without having to switch package.
if all the code is in the same file, then change it.
sed -i 's/old_procedure/new_procedure/g codefile
Is this what you mean?
Is there a way to load entire modules during runtime in Perl? I had thought I found a good solution with autouse but the following bit of code fails compilation:
package tryAutouse2;
use autouse 'tryAutouse';
my $obj = tryAutouse->new();
I imagine this is because autouse is specifically meant to be used with exported functions, am I correct? Since this fails compilation, is it impossible to have a packaged solution? Am I forced to require before each new module invocation if I want dynamic loading?
The reasoning behind this is that my team loads many modules, but we're afraid this is eating up memory.
You want Class::Autouse or ClassLoader.
Due to too much magic, I use ClassLoader only in my REPL for convenience. For serious code, I always load classes explicitely. Jack Maney points out in a comment that Module::Load and Module::Load::Conditional are suitable for delayed loading.
There's nothing wrong with require IMO. Skip the export of the function and just call the fully qualified name:
require Some::Module;
Some::Module::some_function(#some_arguments);
eval 'use tryAutouse; 1;' or die $#;
Will work. But you might want to hide the ugliness.
When you say:
use Foo::Bar;
You're loading module Foo::Bar in at compile time. Thus, if you want to load your module in at run time, you'd use require:
require Foo::Bar;
They are sort of equivalent, but there are differences. See the Perldoc on use to understand the complete difference. For example, require used in this way won't automatically load in imported functions. That might be important to you.
If you want to test whether a module is there or not, wrap up your require statement in an eval and test whether or not eval is successful.
I use a similar technique to see if a particular Perl module is available:
eval { require Mail::Sendmail; };
if ($#) {
$watch->_Send_Email_Net_SMTP($watcher);
return;
}
In the above, I'll attempt to use Mail::Sendmail which is an optional module if it's available. If not, I'll run another routine that uses Net::SMTP:
sub _Send_Email_Net_SMTP {
my $self = shift;
my $watcher = shift;
require Net::SMTP; #Standard module: It should be available
WORD O'WARNING: You need to use curly braces around your eval statement and not parentheses. Otherwise, if the require doesn't work, your program will exit which is probably not what you want to do.
Instruction 'use' is performed at compile time, so check the path to the module also takes place at compile time. This may cause incorrect behavior, which are difficult to understand until you consider the contents of the #INC array.
One solution is to add block 'BEGIN', but the solution shown below is inelegant.
BEGIN { unshift #INC, '/path/to/module/'; }
use My::Module;
You can replace the whole mess a simple directive:
use lib '/path/to/module';
use My::Module;
This works because it is performed at compile time. So everything is ready to execute 'use' instruction.
Instead of the 'BEGIN' block, you can also decide to different instruction executed at compile time ie declaring a constant.
use constant LIB_DIR => '/path/to/module';
use lib LIB_DIR;
use My::Module;
I have a question about packages, modules and syntax. When you access a package in the same file I notice you'd use something like....
package test;
$testVar="This is a test";
#and to access it using..
package main;
print($test::testVar);
or simply...
package test;
print($testVar);
Yet when I use this syntax for use with a module, sending an argument, I am supposed to ommit the $ from the start of the print function, yet above I don't. I noticed it didn't work otherwise and I don't know why. My materials don't clarify.
require module;
print(package::sub("argument"));
Why is that?. I'm confused.
The dollar sign here is a sigil that indicates that the named variable is a scalar.
If there is no preceding package declaration, the use of $var_name contains an implied namespace of main, i.e. it is short for $main::var_name. In the case of your example, where you have package main; first, you need to stipulate that the namespace is test, rather than main, so $test::testVar is required.
For a function call, you do not need to use a sigil. If you did, you would use the ampersand (&), but using ampersands when calling functions has fallen out of favour with many programmers.*
As before, sub_name() is a shortened version of main::sub_name()... in the same way, no sigil is needed to call package::sub().
*References as to the use of &:
Perl Monks discussion
Perl::Critic dislikes it, following on from Perl Best Practices
Per perldoc, using & with a function name allows you to deviate from the usual behaviour. This can result in subtle bugs.