Looking into the Const::Fast source I noticed that it used the built-in function Internals::SvREADONLY internally. Is it safe to use that function directly in my Perl script? It seems to be present in core from Perl 5.8.
my $PI = 4 * atan2 1, 1;
Internals::SvREADONLY($PI => 1);
$PI = 2.718; # Modification of a read-only value attempted at ..
C:\>perldoc Internals
No documentation found for "Internals".
No.
More specifically, the package is named "Internals" for a reason. It is not intended for use outside the core. It could change without notice.
This is not quite answering your question, but I think it is worth mentioning so others don't experience the same pain as I have: don't use any readonly value if you're running on a version of Perl earlier than 5.10.1. Consider this little example:
{
package Foo;
sub foo { print "I'm in foo!\n"; }
}
use strict;
use warnings;
use Readonly;
Readonly my #classes => qw(Foo);
foreach my $class (#classes)
{
# this dies with "Can't call method "foo" without a package or object reference"
$class->foo;
}
Since my XS-fu is not very high, I can't explain what is going on here very coherently (but Devel::Peek shows some interesting things in the $class variable).
Related
Is it possible to create global functions available across all namespaces like perl built-in functions?
First of all, "function" is the name given to Perl's named list operators, named unary operators and named nullary operators. They are visible everywhere because they are operators, just like ,, && and +. Subs aren't operators.
Second of all, you ask how to create a global sub, but all subs are already global (visible from everywhere) in Perl! You simply need to quality the name of the sub with the package if it's not in the current package. For example, Foo::mysub() will call my_sub found in package Foo from anywhere.
But maybe you want to be able to say mysub() instead of Foo::mysub() from everywhere, and that's a very bad idea. It violates core principles of good programming. The number of types of problems it can cause are too numerous to list.
There is a middle ground. A better solution is to create a sub that can be imported into the namespaces you want. For example, say you had the module
package Foo;
use Exporter qw( import );
our #EXPORT_OK = qw( my_sub );
our %TAGS = ( ALL => \#EXPORT_OK );
sub my_sub { ... }
1;
Then, you can use
use Foo qw( my_sub );
to load the module (if it hasn't already been loaded) and create my_sub in the current package. This allows it to call the sub as my_sub() from the package into which it was imported.
There is nothing simple that would allow one to somehow "register" user's subs with the interpreter, or some such, so that you could run them as builtins in any part of the program.
One way to get the behavior you ask for is to directly write to symbol tables of loaded modules. This has to be done after the modules have been loaded, and after subs that you add to those modules have been defined. I use INIT block in the example below.
Note that this has a number of weaknesses and just in general the idea itself is suspect to me, akin to extending the interpreter. Altogether I'd much rather write a module with all such subs and use standard approaches for good program design to have that module loaded where it needs to go.
Having said that, here is a basic demo
use warnings;
use strict;
use feature 'say';
use Data::Dump qw(dd pp);
use TestMod qw(modsub);
sub t_main { say "In t_main(), from ", __PACKAGE__ }
modsub("Calling from main::");
INIT {
no strict 'refs';
foreach my $pkg (qw(TestMod)) {
*{ $pkg . '::' . 'sub_from_main' } = \&t_main;
}
dd \%TestMod::;
}
This copies the reference to t_main from the current package (main::) into the symbol table of $pkg, under the name of sub_from_main, which can then be used with that name in that package.
For simplicity the name of the module is hardcoded, but you can use %INC instead, and whatever other clues you have, to figure out what loaded modules' stashes to add to.
The benefactor (or the victim?) module TestMod.pm
package TestMod;
use warnings;
use strict;
use feature 'say';
use Exporter qw(import);
our #EXPORT_OK = qw(modsub);
sub modsub {
say "In module ", __PACKAGE__, ", args: #_";
say "Call a sub pushed into this namespace: ";
sub_from_main();
}
1;
The name of the added sub can be passed to modules as they're loaded, instead of being hardcoded, in which case you need to write their import sub instead of borrowing the Exporter's one.
There are also modules that allow one to add keywords, but that's no light alternative.
The answer seems to be no, but you can impliment most of the behaivior that you want by using the symbol table *main::main:: to define a subroutine in all the namespaces.
use strict;
use warnings;
use Data::Dump qw(dd);
my $xx = *main::main::;
package A;
sub test {
printf "A::%s\n", &the_global;
}
package B;
sub the_global
{
"This is B::the_global";
}
sub test {
printf "B::%s\n", &the_global;
}
package main;
my $global_sub = sub { "The Global thing" };
for my $NS (keys %$xx) {
if ($NS =~ /^[A-Z]::$/) {
my $x = $NS . 'the_global';
if (defined &$x) {
printf "Skipping &%s\n", $x;
} else {
printf "Adding &%s\n", $x;
no strict 'refs';
*$x = $global_sub;
}
}
}
A::test;
This will not work on packages that are not referenced at all before the for loop above is run. But this would only happen if a require, use or package was eval'd after the code started running.
This is also still a compiler issue! You either need to refer to the global function as the_global() or &the_global if you are (as you should be) using use strict.
Sorry for my late response and thank you all for yours detailed answers and explanations.
Well.. I understood the right answer is: IT'S NOT POSSIBLE!
I'm mantaining a Perl framework used by some customers, and that framework exports some specialized subs (logging, event handling, controllers for hardware devices, domain specific subs and so). That's why I tried to figure out how to prevent the developers from importing my subs in all their packages.
I have Perl code which relies on Term::ReadKey to get the terminal width. My installation is missing this module, so I want to provide a default if the module isn't present rather than throw an exception.
How can I conditionally use an optional module, without knowing ahead of time whether it is available.
# but only if the module is installed and exists
use Term::ReadKey;
...
How can I accomplish this?
Here's a bare-bones solution that does not require another module:
my $rc = eval
{
require Term::ReadKey;
Term::ReadKey->import();
1;
};
if($rc)
{
# Term::ReadKey loaded and imported successfully
...
}
Note that all the answers below (I hope they're below this one! :-) that use eval { use SomeModule } are wrong because use statements are evaluated at compile time, regardless of where in the code they appear. So if SomeModule is not available, the script will die immediately upon compiling.
(A string eval of a use statement will also work (eval 'use SomeModule';), but there's no sense parsing and compiling new code at runtime when the require/import pair does the same thing, and is syntax-checked at compile time to boot.)
Finally, note that my use of eval { ... } and $# here is succinct for the purpose of this example. In real code, you should use something like Try::Tiny, or at least be aware of the issues it addresses.
Check out the CPAN module Module::Load::Conditional. It will do what you want.
The classic answer (dating back to Perl 4, at least, long before there was a 'use') was to 'require()' a module. This is executed as the script is run, rather than when compiled, and you can test for success or failure and react appropriately.
if (eval {require Term::ReadKey;1;} ne 1) {
# if module can't load
} else {
Term::ReadKey->import();
}
or
if (eval {require Term::ReadKey;1;}) {
#module loaded
Term::ReadKey->import();
}
Note: the 1; only executes if require Term::... loaded properly.
And if you require a specific version of the module:
my $GOT_READKEY;
BEGIN {
eval {
require Term::ReadKey;
Term::ReadKey->import();
$GOT_READKEY = 1 if $Term::ReadKey::VERSION >= 2.30;
};
}
# elsewhere in the code
if ($GOT_READKEY) {
# ...
}
use Module::Load::Conditional qw(check_install);
use if check_install(module => 'Clipboard') != undef, 'Clipboard'; # class methods: paste, copy
using if pragma and Module::Load::Conditional core module.
check_install returns hashref or undef.
this module is also mentioned in the see also section of the pragma's documentation:
Module::Load::Conditional provides a number of functions you can use to query what modules are available, and then load one or more of them at runtime.
This is an effective idiom for loading an optional module (so long as you're not using it in a code base with sigdie handler),
use constant HAS_MODULE => defined eval { require Module };
This will require the module if available, and store the status in a constant.
You can use this like,
use constant HAS_READLINE => defined eval { require Term::ReadKey };
my $width = 80;
if ( HAS_READLINE ) {
$width = # ... code, override default.
}
Note, if you need to import it and bring in the symbols you can easily do that too. You can follow it up.
use constant HAS_READLINE => defined eval { require Term::ReadKey };
Term::ReadKey->import if HAS_READLINE;
This method uses constants. This has the advantage that if you don't have this module the dead code paths are purged from the optree.
I think it doesn't work when using variables.
Please check this link which explains how it can be used with variable
$class = 'Foo::Bar';
require $class; # $class is not a bareword
#or
require "Foo::Bar"; # not a bareword because of the ""
The require function will look for the "Foo::Bar" file in the #INC array and will complain about not finding "Foo::Bar" there. In this case you can do:
eval "require $class";
this is my first foray into subclassing with perl and I am wondering why I am getting this simple error...
"Can't locate object method "prepare" via package "WebDB::st" at /home/dblibs/WebDB.pm line 19.". It seems to find the module WebDB ok, but not the prepare subroutine in ::st
First here's my package (both packages are in one file, WebDB.pm)
package WebDB;
use strict;
use DBI;
sub connect {
my $dbh = (DBI->connect ("DBI:mysql:test:127.0.0.1", "root","",
{ PrintError => 1, RaiseError => 0 }));
return bless $dbh, 'WebDB::st';
}
package WebDB::st;
our #ISA = qw(::st);
sub prepare {
my ($self, $str, #args) = #_;
$self->SUPER::prepare("/* userid:$ENV{USER} */ $str", #args);
}
1;
I also tried replacing the "our #ISA = qw(;;st)" with "use base 'WebDB'" and same problem.
I'm thinking it's probably something very simple that I'm overlooking. Many thanks! Jane
Subclassing DBI has to be done just right to work correctly. Read Subclassing the DBI carefully and properly set RootClass (or explicitly call connect on your root class with #ISA set to DBI). Make sure you have WebDB::st subclassing DBI::st and a WebDB::db class subclassing DBI::db (even if there are no methods being overridden). No need to rebless.
Avoid using base; it has some unfortunate behavior that has led to its deprecation, particularly when used with classes that are not in a file of their own.
Either explicitly set #ISA or use the newer parent pragma:
package WebDB;
use parent 'DBI';
...
package WebDB::db;
use parent -norequire => 'DBI::db';
...
package WebDB::st;
use parent -norequire => 'DBI::st';
...
Are WebDB and WebDB::st in one file or two? If they are in separate files, I don't see anything that is doing a use WebDB::st;, which would cause that file to be loaded.
You can do either of these things as a remedy -- put the two packages in the same file (that would look exactly as you have pasted it above), or add a use WebDB::st; line in WebDB.pm.
(I'd also add use strict; use warnings; in both these packages too.)
Also, the prepare function is not in ::st -- there is no such package (unless it is defined elsewhere). prepare is in the WebDB::st namespace -- via the package declaration. You are however declaring that WebDB::st has ::st as a parent.
If subclassing is as tricky as ysth seems to think, I might recommend Class::Delegator from CPAN. I use if for classes that want to act like IO. And by it, Perl is the first language (that I am aware of) that has an expression language for aggregation, delegation, encapsulation almost equal with inheritance.
package WebDB;
use strict;
use DBI;
use Class::Delegator
send => [ qw<connect ...> ]
, to => '{_dbihandle}'
...
;
Lets say you have a parent Perl class in one file:
#!/usr/bin/perl
package Foo;
use strict;
use warnings;
use Data::Dumper;
sub new{
my $class = shift;
my %self = ();
return bless %self, $class;
}
1;
and a subclass in a different file:
#!/usr/bin/perl
package Bar;
use base "Foo";
1;
Will the subclass inherit the use statements from the parent? I know the method new will be inherited.
Basically I am trying to reduce the amount of boilerplate in my code and I can't find a clear answer to this question.
You asked in a comment about Test::Most and how it reduces boilerplate. Look at its import method. It's loading the modules into its namespace, adding those symbols to #EXPORT, then re-calling another import through a goto to finally get them into the calling namespace. It's some serious black magic that Curtis has going on there, although I wonder why he just didn't use something like import_to_level. Maybe there are some side effects I'm not thinking about.
I talk quite a bit about this sort of thing in Avoid accidently creating methods from module exports in The Effective Perler. It's in a different context but it's some of the same issues.
Here's a different example.
If some other module loads a module, you have access to it. It's not good to depend on that though. Here are three separate files:
Top.pm
use 5.010;
package Top;
use File::Spec;
sub announce { say "Hello from top!" }
1;
Bottom.pm
package Bottom;
use parent qw(Top);
sub catfiles { File::Spec->catfile( #_ ) }
1;
test.pl
use 5.010;
use Bottom;
say Bottom->catfiles( qw(foo bar baz) );
say File::Spec->catfile( qw( one two three ) );
I only load File::Spec in Top.pm. However, once loaded, I can use it anywhere in my Perl program. The output shows that I was able to "use" the module in other files even though I only loaded it in one:
Bottom/foo/bar/baz
one/two/three
For this to work, the part of the code that loads the module has to load before any other part of the code tries to use that module. As I said, it's a bad idea to depend on this: things break if the loading sequence changes or the loading module disappears.
If you want to import symbols, however, you have to explicitly load the module you want while you are in the package you want to import into. That's just so the exporting module defines the symbols in that package. It's not something that depends with scope.
Ah, good question!
Will the subclass inherit the use statements from the parent?
Well this depends on what you mean by inherit. I won't make any assumptions until the end, but the answer is maybe. You see, perl mixes the ideas of Classes, and Namespaces -- a package is a term that can describe either of them. Now the issue is the statement use all it does is force a package inclusion, and call the targets import() sub. This means it essentially has unlimited control over your package - and by way of that your class.
Now, compound this with all methods in perl being nothing more than subs that take $self as a first argument by convention and you're left with perl5. This has an enormous upside for those that know how to use it. While strict is a lexical pragma, what about Moose?
package BigMooseUser;
use Moose;
package BabyMooseUser;
our #ISA = 'BigMooseUser';
package Foo;
my $b = BabyMooseUser->new;
print $b->meta->name;
Now, where did BabyMooseUser get the constructor (new) from? Where did it get the meta class from? All of this is provided from a single use Moose; in the parent class (namespace). So
Will the subclass inherit the use statements from the parent?
Well, here, in our example, if the effects of the use statement are to add methods, than certainly.
This subject is kind of deep, and it depends if you're talking about pragmas, or more obscure object frameworks, or procedural modules. If you want to mitigate a parents namespace from affecting your own in the OO paradigm see namespace::autoclean.
For boilerplate reduction, I have a couple of strategies: Most of my classes are Moose classes, which takes care of OO setup and also gives me strict and warnings. If I want to have functions available in many packages, I'll create a project specific MyProject::Util module that uses Sub-Exporter to provide me with my own functions and my own interface. This makes it more consistent, and if I decide to change the Dumper (for example) later for whatever reason, I don't have to change lots of code. That'll also allow you to group exports. A class then usually looks something like this:
package Foo;
use Moose;
use MyProject::Util qw( :parsing :logging );
use namespace::autoclean;
# class implementation goes here
1;
If there's other things you regard as boilerplate and want to make simpler to include, it of course depends on what those things are.
A pragmatic answer to your problem: Either use, or look at how Modern::Perl does it to enforce strict and warnings.
You can get a definitive answer by examining the symbol tables for each package:
# examine-symbol-tables.pl
use Bar;
%parent_names = map{$_ => 1} keys %Foo::;
%child_names = map{$_ => 1} keys %Bar::;
delete $parent_names{$_} && ($common_names{$_} = delete $child_names{$_}) foreach keys %child_names;
print "Common names in symbol tables:\n";
print "#{[keys %common_names]}\n\n";
print "Unique names in Bar symbol table:\n";
print "#{[keys %child_names]}\n\n";
print "Unique names in Foo symbol table:\n";
print "#{[keys %parent_names]}\n\n";
$ perl inherit.pl
Common names in symbol tables:
BEGIN
Unique names in Bar symbol table:
ISA isa import
Unique names in Foo symbol table:
Dumper new VERSION
How do I list available methods on a given object or package in Perl?
There are (rather too) many ways to do this in Perl because there are so many ways to do things in Perl. As someone commented, autoloaded methods will always be a bit tricky. However, rather than rolling your own approach I would suggest that you take a look at Class::Inspector on CPAN. That will let you do something like:
my $methods = Class::Inspector->methods( 'Foo::Class', 'full', 'public' );
If you have a package called Foo, this should do it:
no strict 'refs';
for(keys %Foo::) { # All the symbols in Foo's symbol table
print "$_\n" if exists &{"Foo::$_"}; # check if symbol is method
}
use strict 'refs';
Alternatively, to get a list of all methods in package Foo:
no strict 'refs';
my #methods = grep { defined &{"Foo::$_"} } keys %Foo::;
use strict 'refs';
if you have a package that is using Moose its reasonably simple:
print PackageNameHere->meta->dump;
And for more complete data:
use Data::Dumper;
print Dumper( PackageNameHere->meta );
Will get you started. For everything else, theres the methods that appear on ->meta that are documented in Class::MOP::Class
You can do a bit of AdHoc faking of moose goodness for packages without it with:
use Class::MOP::Class;
my $meta = Class::MOP::Class->initialize( PackageNameHere );
and then proceed to use the Class::MOP methods like you would with Moose.
For starters:
$meta->get_method_map();
use Moose; #, its awesome.
In general, you can't do this with a dynamic language like Perl. The package might define some methods that you can find, but it can also make up methods on the fly that don't have definitions until you use them. Additionally, even calling a method (that works) might not define it. That's the sort of things that make dynamic languages nice. :)
What task are you trying to solve?
A good answer from How to get structure and inheritance history
The classes from which an object's class currently inherits can be found using the following:
use mro qw( );
use Scalar::Util qw( blessed );
say join ", ", #{ mro::get_linear_isa(blessed($o)) };