I had exported some constants from my module. In my script I am loading my module using Mojo::Loader
my module
use constant FALSE => 0;
use constant TRUE => 1;
our #EXPORT = qw(FALSE TRUE);
In my script.
Mojo::Loader->new->load($my_module_name);
I was able to use my module in my script, but the constants that I exported were not accessible in my script. If I load my module with use clause. I was able to use the exported constants.
Any idea how to fix this and import the constants into my script.
Thanks!!
I took a look at the code for Mojo::Loader and it turns out it cannot import stuff. It only does a require (in a string eval), but not a use. A quick grep of the source reveals that there is no import whatsoever, so you will need to call Your::Module->import yourself.
Here's a link to the relevant part of the source code and a quote:
sub load {
my ($self, $module) = #_;
# Check module name
return 1 if !$module || $module !~ /^\w(?:[\w:']*\w)?$/;
# Load
return undef if $module->can('new') || eval "require $module; 1";
# Exists
return 1 if $# =~ /^Can't locate \Q#{[class_to_path $module]}\E in \#INC/;
# Real error
return Mojo::Exception->new($#);
}
There is something interesting going on here. If you use foo, the import works with constants.
use foo;
print 'True: ', TRUE;
However:
require foo;
foo->import;
print 'True: ', TRUE;
This will produce a warning Bareword "TRUE" not allowed while "strict subs" in use. So we put TRUE() to make it look less like a bareword. A constant is a sub after all. Now it will work. The same goes for doing Mojo::Loader->load('foo').
If you wrap a BEGIN block around the require and import, you can omit the parenthesis.
Thus, if you want to export constants, add parenthesis to where you call them if you want to keep using Mojo::Loader.
Related
th_foo.pm
package Foo;
use Exporter 'import';
our #EXPORT = 'foo';
sub foo { print "foo\n"; }
sub bar { print "bar\n"; }
1;
Case 1
main.pl
use lib '.';
delete $INC{ 'th_foo.pm' };
require 'th_foo.pm';
'Foo'->import();
foo();
Result
$ perl main.pl
foo
Case 2
main.pl
use lib '.';
delete $INC{ 'th_foo.pm' };
require 'th_foo.pm';
'th_foo.pm'->import();
foo();
Result
$ perl main.pl
Undefined subroutine &main::foo called at th.pl line 9.
Question
Can we call Foo's import method by only knowing the file name, th_foo.pm, and not knowing the package name, Foo?
There is no way to get the packages used by an executed file.
You could scan the symbol table beforehand and afterwards and check for differences, but that will also include symbols created by modules used by the file you are executing.
If you can control the th_foo.pm file, you can use the last evaluated expression to pass back information through the require. This only works the first time you require the file in a program, but you're already playing the %INC.
package Foo;
use Exporter 'import';
our #EXPORT = 'foo';
sub foo { print "foo\n"; }
sub bar { print "bar\n"; }
__PACKAGE__;
Then, in the program:
my $package = require 'th_foo.pm';
$package->import;
Again, this is fragile because you must be the first require on that file (otherwise it just returns a boolean). But, in one case I have done something like this, which is sure to get you a beating in code review:
sub my_require ( $file ) {
state %cache;
my $rc = require $file;
$cache{$file} //= $rc;
}
I do this for all of my Makefile.PL files in my Perl dists (example in object-iterate). When I want to process all of my distributions to do something (like, which ones have a minimum version of v5.8?), I require the Makefile.PL and know which package I'm dealing with (although the repo path works too but needs more work).
Curiously, there is a suggestion that we should remove the true value from the end of Perl modules. David Farrell wrote about it in Patching Perl: loading modules that return false and it's now an issue for Perl 7. This doesn't affect the return value though; it just doesn't complain if you return false or empty. It knows it failed through other means.
is it possible to import (use) a perl module within a different namespace?
Let's say I have a Module A (XS Module with no methods Exported #EXPORT is empty) and I have no way of changing the module.
This Module has a Method A::open
currently I can use that Module in my main program (package main) by calling A::open I would like to have that module inside my package main so that I can directly call open
I tried to manually push every key of %A:: into %main:: however that did not work as expected.
The only way that I know to achieve what I want is by using package A; inside my main program, effectively changing the package of my program from main to A.
Im not satisfied with this. I would really like to keep my program inside package main.
Is there any way to achieve this and still keep my program in package main?
Offtopic: Yes I know usually you would not want to import everything into your namespace but this module is used by us extensively and we don't want to type A:: (well the actual module name is way longer which isn't making the situation better)in front of hundreds or thousands of calls
This is one of those "impossible" situations, where the clear solution -- to rework that module -- is off limits.
But, you can alias that package's subs names, from its symbol table, to the same names in main. Worse than being rude, this comes with a glitch: it catches all names that that package itself imported in any way. However, since this package is a fixed quantity it stands to reason that you can establish that list (and even hard-code it). It is just this one time, right?
main
use warnings;
use strict;
use feature 'say';
use OffLimits;
GET_SUBS: {
# The list of names to be excluded
my $re_exclude = qr/^(?:BEGIN|import)$/; # ...
my #subs = grep { !/$re_exclude/ } sort keys %OffLimits::;
no strict 'refs';
for my $sub_name (#subs) {
*{ $sub_name } = \&{ 'OffLimits::' . $sub_name };
}
};
my $name = name('name() called from ' . __PACKAGE__);
my $id = id('id() called from ' . __PACKAGE__);
say "name() returned: $name";
say "id() returned: $id";
with OffLimits.pm
package OffLimits;
use warnings;
use strict;
sub name { return "In " . __PACKAGE__ . ": #_" }
sub id { return "In " . __PACKAGE__ . ": #_" }
1;
It prints
name() returned: In OffLimits: name() called from main
id() returned: In OffLimits: id() called from main
You may need that code in a BEGIN block, depending on other details.
Another option is of course to hard-code the subs to be "exported" (in #subs). Given that the module seems to be immutable in practice this option is reasonable and more reliable.
This can also be wrapped in a module, so that you have the normal, selective, importing.
WrapOffLimits.pm
package WrapOffLimits;
use warnings;
use strict;
use OffLimits;
use Exporter qw(import);
our #sub_names;
our #EXPORT_OK = #sub_names;
our %EXPORT_TAGS = (all => \#sub_names);
BEGIN {
# Or supply a hard-coded list of all module's subs in #sub_names
my $re_exclude = qr/^(?:BEGIN|import)$/; # ...
#sub_names = grep { !/$re_exclude/ } sort keys %OffLimits::;
no strict 'refs';
for my $sub_name (#sub_names) {
*{ $sub_name } = \&{ 'OffLimits::' . $sub_name };
}
};
1;
and now in the caller you can import either only some subs
use WrapOffLimits qw(name);
or all
use WrapOffLimits qw(:all);
with otherwise the same main as above for a test.
The module name is hard-coded, which should be OK as this is meant only for that module.
The following is added mostly for completeness.
One can pass the module name to the wrapper by writing one's own import sub, which is what gets used then. The import list can be passed as well, at the expense of an awkward interface of the use statement.
It goes along the lines of
package WrapModule;
use warnings;
use strict;
use OffLimits;
use Exporter qw(); # will need our own import
our ($mod_name, #sub_names);
our #EXPORT_OK = #sub_names;
our %EXPORT_TAGS = (all => \#sub_names);
sub import {
my $mod_name = splice #_, 1, 1; # remove mod name from #_ for goto
my $re_exclude = qr/^(?:BEGIN|import)$/; # etc
no strict 'refs';
#sub_names = grep { !/$re_exclude/ } sort keys %{ $mod_name . '::'};
for my $sub_name (#sub_names) {
*{ $sub_name } = \&{ $mod_name . '::' . $sub_name };
}
push #EXPORT_OK, #sub_names;
goto &Exporter::import;
}
1;
what can be used as
use WrapModule qw(OffLimits name id); # or (OffLimits :all)
or, with the list broken-up so to remind the user of the unusual interface
use WrapModule 'OffLimits', qw(name id);
When used with the main above this prints the same output.
The use statement ends up using the import sub defined in the module, which exports symbols by writing to the caller's symbol table. (If no import sub is written then the Exporter's import method is nicely used, which is how this is normally done.)
This way we are able to unpack the arguments and have the module name supplied at use invocation. With the import list supplied as well now we have to push manually to #EXPORT_OK since this can't be in the BEGIN phase. In the end the sub is replaced by Exporter::import via the (good form of) goto, to complete the job.
You can forcibly "import" a function into main using glob assignment to alias the subroutine (and you want to do it in BEGIN so it happens at compile time, before calls to that subroutine are parsed later in the file):
use strict;
use warnings;
use Other::Module;
BEGIN { *open = \&Other::Module::open }
However, another problem you might have here is that open is a builtin function, which may cause some problems. You can add use subs 'open'; to indicate that you want to override the built-in function in this case, since you aren't using an actual import function to do so.
Here is what I now came up with. Yes this is hacky and yes I also feel like I opened pandoras box with this. However at least a small dummy program ran perfectly fine.
I renamed the module in my code again. In my original post I used the example A::open actually this module does not contain any method/variable reserved by the perl core. This is why I blindly import everything here.
BEGIN {
# using the caller to determine the parent. Usually this is main but maybe we want it somewhere else in some cases
my ($parent_package) = caller;
package A;
foreach (keys(%A::)) {
if (defined $$_) {
eval '*'.$parent_package.'::'.$_.' = \$A::'.$_;
}
elsif (%$_) {
eval '*'.$parent_package.'::'.$_.' = \%A::'.$_;
}
elsif (#$_) {
eval '*'.$parent_package.'::'.$_.' = \#A::'.$_;
}
else {
eval '*'.$parent_package.'::'.$_.' = \&A::'.$_;
}
}
}
I've got a Perl script that I'm trying to make compatible with two different Perl environments. To work around the two different versions of Socket I have, I'm doing a little hackery with require and import. I've got it working, but I'm not happy with the behavior.
Mod.pm:
package Mod;
use base 'Exporter';
our #EXPORT = qw( MAGIC_CONST );
sub MAGIC_CONST() { 42; }
test.pl:
use Mod;
#require Mod;
#import Mod;
printf "MAGIC_CONST = ". MAGIC_CONST ."\n";
printf "MAGIC_CONST = ". MAGIC_CONST() ."\n";
Outputs:
MAGIC_CONST = 42
MAGIC_CONST = 42
But using the 'require' and 'import' instead, I get this:
Output:
MAGIC_CONST = MAGIC_CONST
MAGIC_CONST = 42
So the question is: Is there a clean way I can get the normal behavior of the constants? I can certainly do sub MAGIC_CONST { Mod::MAGIC_CONST(); } but that's pretty ugly.
What I'm actually doing is something like this:
use Socket;
if ($Socket::VERSION > 1.96) {
import Socket qw(SO_KEEPALIVE); # among others
setsockopt($s, SOL_SOCKET, SO_KEEPALIVE); # among others
}
The reason the require version prints MAGIC_CONST instead of 42 is because use is what tells perl to import the symbols from one module to another. Without the use, there is no function called MAGIC_CONST defined, so perl interprets it as a string instead. You should use strict to disable the automatic conversion of barewords like that into strings.
#!/usr/bin/env perl
no strict;
# forgot to define constant MAGIC_CONST...
print 'Not strict:' . MAGIC_CONST . "\n";
produces
Not strict:MAGIC_CONST
But
#!/usr/bin/env perl
use strict;
# forgot to define constant MAGIC_CONST...
print 'Strict:' . MAGIC_CONST . "\n";
Produces an error:
Bareword "MAGIC_CONST" not allowed while "strict subs" in use at
./test.pl line 4. Execution of ./test.pl aborted due to compilation
errors.
So if you want to use one module's functions in another, you either have to import them with use, or call them with the full package name:
package Foo;
sub MAGIC_CONST { 42 };
package Bar;
print 'Foo from Bar: ' . Foo::MAGIC_CONST . "\n";
Foo from Bar: 42
It's usually best to avoid conditionally importing things. You could resolve your problem as follows:
use Socket;
if ($Socket::VERSION > 1.96) {
setsockopt($s, SOL_SOCKET, Socket::SO_KEEPALIVE);
}
If you truly want to import, you still need to do it at compile-time.
use Socket;
use constant qw( );
BEGIN {
if ($Socket::VERSION > 1.96) {
Socket->import(qw( SO_KEEPALIVE ));
} else {
constant->import({ SO_KEEPALIVE => undef });
}
}
setsockopt($s, SOL_SOCKET, SO_KEEPALIVE) if defined(SO_KEEPALIVE);
Adam's answer gives a good explanation of what is going on and how to get the desired behavior. I am going to recommend not using constant.pm to define symbolic names for constants. A fairly nice looking and convenient alternative with fewer gotchas is to use constants defined using Const::Fast and allow them to be imported.
In addition, by using Importer in the module which wants to import the constants, the module that defines the constants can avoid inheriting Exporter's heavy baggage, or having to use Exporter's import.
The fact that Const::Fast allows you to define real constant arrays and hashes is a bonus.
For example:
package MyConstants;
use strict;
use warnings;
use Const::Fast;
use Socket;
const our #EXPORT => ();
const our #EXPORT_OK => qw(
%SOCKET_OPT
);
const our %SOCKET_OPT => (
keep_alive => ($Socket::VERSION > 1.96) ? Socket::SO_KEEPALIVE : undef,
);
__PACKAGE__;
__END__
Using these constants in a script:
#!/usr/bin/env perl
use strict;
use warnings;
use Socket;
use Importer 'MyConstants' => qw( %SOCKET_OPT );
if ( defined $SOCKET_OPT{keep_alive} ) {
setsockopt($s, SOL_SOCKET, $SOCKET_OPT{keep_alive});
}
As I note in my blog post:
f I want to read the constants from a configuration file, that's trivial. If I want to export them to JSON, YAML, INI, or whatever else, that's also trivial. I can interpolate them willy-nilly.
For those who have taken seriously Exporter's stance against exporting variables, this takes some getting used to. Keep in mind that the admonition is there to make sure you don't write code that willy nilly modifies global variables. However, in this case, the variables we are exporting are not modifiable. Maybe you can convince yourself that the concern really does not apply in this instance. If you try to refer to a non-existent constant, you get an error (albeit during run time) ...
In most cases, the benefits of this approach outweigh the speed penalty of not using constant.pm.
How to pass some arguments while using a package, for example:
use Test::More tests => 21;
I wasn't able to find any valuable documentation about this featue. Are there any pros and cons of passing such arguments?
use My::Module LIST does two things: 1) It requires My::Module; and 2) Invokes My::Module->import(LIST).
Therefore, you can write your module's import routine to treat the list of arguments passed any which way you want. This becomes even easier if you are indeed writing an object oriented module that does not export anything to the caller's namespace.
Here's a rather pointless example:
package Ex;
use strict;
use warnings;
{
my $hello = 'Hello';
sub import {
my $self = shift;
my $lang = shift || 'English';
if ($lang eq 'Turkish') {
$hello = 'Merhaba';
}
else {
$hello = 'Hello';
}
return;
}
sub say_hello {
my $self = shift;
my $name = shift;
print "$hello $name!\n";
return;
}
}
__PACKAGE__;
__END__
And a script to use it:
#!/usr/bin/env perl
use strict;
use warnings;
use Ex 'Turkish';
Ex->say_hello('Perl');
Ex->import;
Ex->say_hello('Perl');
Output:
$ ./imp.pl
Merhaba Perl!
Hello Perl!
Some may say it is more readable in some scenarios, but in essence it is same as
use Test::More qw(tests 21);
(test is auto-quoted by fat comma =>, and number doesn't need quote).
The major disadvantage is that you can't use the default import subroutine from Exporter, which expects only a list of symbols (or tags denoting collections of symbols) to import into the calling package
Test::More inherits a custom import routine from the superclass Test::Builder::Module, which uses the arguments supplied in the use statement to configure the test plan. It also in turn uses Exporter to handle options specified like import => [qw/ symbols to import /]
Pretty much anything can be done by a custom import subroutine if you have a specific requirement, but it is probably unwise to stray too far from standard object-oriented semantics
Having a C background, I may be trying to write something the wrong way so excuse the beginner question. Here is what I'm looking for :
I'm willing to have a perl module Verbose (not a class) that define a subroutine called verbose_print(). This subroutine will print its argument (a string) only if module's variable $verbose is true. Of course, what I'm willing to do is to get the -V option from the command line (using Getopt::Long) and then, is the -V switch is on, call the Verbose module with 'true' being the value for $Verbose::verbose.
One workaround is to have a verbose_init function that set the $Verbose::verbose variable to true inside the Verbose module.
Another one was to declare $verbose in my module using our and then $Verbose::verbose = $command_line_verbose_switch in the main script.
I was wondering if there is another way to do this in perl?
Don't be so afraid of classes in Perl, they're just packages and modules treated a wee bit differently. They don't bite. However, you said no classes, so no classes.
I prefer not to touch package variables directly. Instead, I'll use a subroutine to set them.
Here's my Local::Verbose (stored under Local/Verbose.pm)
package Local::Verbose;
use strict;
use warnings;
use Exporter 'import';
# Could have used just '#EXPORT', but that's bad manners
our #EXPORT_OK = qw(verbose verbose_switch);
# Use "our", so $verbose_value is a package variable.
# This makes it survive between calls to this package
our $verbose_value;
# prints out message, but only if $verbose_value is set to non-blank/zero value
sub verbose {
my $message = shift;
if ( $verbose_value ) {
print "VERBOSE: $message\n";
return $message;
}
else {
return;
}
}
sub verbose_switch {
my $switch_value = shift;
$verbose_value = $switch_value;
return $switch_value;
}
1;
Notice the our. That makes $verbose_value a package variable which means it lives on outside of the package between calls.
Notice how I use Exporter and the #EXPORT_OK array. You can use #EXPORT which will export all of the named subroutines automatically, but it's now considered bad manners since you could end up covering over someone's local subroutine with the same name. Better make it explicit. If there's a problem, they can use the Local::Verbose::verbose name of the verbose subroutine.
And how it's used
use strict;
use warnings;
use Local::Verbose qw(verbose verbose_switch);
verbose ("This is a test");
verbose_switch(1);
verbose ("This is a second test");
By the way, imagine calling the verbose subroutine like this:
verbose($message, $my_verbose_level);
Now, your verbose subroutine could look like this:
sub verbose {
my $message = shift;
my $verbose_level = shift;
if (not defined $verbose) {
$verbose_level = 1;
}
if ( $verbose_value =< $verbose_level ) {
print "VERBOSE: $message\n";
return $message;
}
else {
return;
}
}
Now, you can set your verbose level to various values, and have your verbose statements give you different levels of verbosity. (I do the same thing, but call it debug).
One of 'another ways' is create an import function:
package Verbose;
my $verbose_on;
sub import {
$verbose_on = shift;
}
#... Your code.
Now you can set your verbose like this:
if( ... ) { #check getopt
use Verbose (1); #This will require package Verbose and call "import"
}
But, i think more simple and obivious to further use is make a function-setter.