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}'
...
;
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 try to intercept nonexistent methods call in some subclass.
Yes, I know about AUTOLOAD,
but (for methods) it try to call parent::method first, then UNIVERSAL::method and only then ::AUTOLOAD.
But I need call (something like) ::AUTOLOAD at first.
Because I want to know what methods subclass try to call from parent.
Give me some advice about it please.
If you just want to know what methods are being used, you can use some profiling module like Devel::NYTProf.
If you want to react to that during your program execution, you can intercept directly the entersub opcode just as the profiling modules do. See the perlguts or profiling module code for more details.
You could probably create a 'Monitor' class with FETCH and EXISTS and tie it to the symbol table hash like: tie %Module::Name:: , Monitor;
But unless we know exactly what you are trying to do and why, it's hard to guess what would be the right solution for you.
Please heavily consider Jiri Klouda's suggestion that you step back and reconsider what you are trying to accomplish. You almost never want to do what you're trying to do.
But, if you're really sure you want to, here's how to get enough pure Perl rope to hang yourself...
The subs pragma takes a list of sub names to predeclare. As tchrist says above, you can predeclare subs but never actually define them. This will short-circuit method dispatch to superclasses and call your AUTOLOAD immediately.
As for the list of sub names to pass to the pragma, you could use Class::Inspector->methods (thanks to Nic Gibson's answer for teaching me about this module).
According to brian d foy's comment to Nic Gibson's answer, Class::Inspector will not handle methods defined in UNIVERSAL. If you need to do those separately, you can get inspiration from the 'use subs' line in my Class::LazyObject module.
Why not create an AUTOLOAD sub in the sub-class package which 1) reports the missing method and then 2) dispatches the call to the parent. For this to work you don't defined #ISA in the sub-class.
Something like:
package my_parent;
sub foo { warn "in my_parent::foo" }
package my_subclass;
my $PARENT_CLASS = "my_parent"; # assume only one parent
# Note: no #ISA defined here
sub AUTOLOAD {
warn "non-existent method $AUTOLOAD called\n";
my $self = shift;
(my $method = $AUTOLOAD) =~ s{.*::}{};
my $super = $PARENT_CLASS . '::' . $method;
$self->$super(#_);
}
package main;
my $x = bless {}, 'my_subclass';
$x->foo;
The syntax: $self->$super(#_) where $super has double-colons in it tells perl in which package to start looking for the method, e.g.:
$self->my_parent::foo(...)
will look for the foo method starting in the package my_parent regarless of what class $self is blessed into.
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
I have a number of different sites that I download data from and massage into other formats (using Perl) for use at work, that are all run from one Perl script kinda like so:
#! /usr/bin/perl
use strict;
use My::Package1;
use My::Package2;
my $p1 = My::Package1->new;
$p1->download;
my $p2 = My::Package2->new;
$p2->download;
and so on and so forth. At the moment each My::Package is its own package; it doesn't inherit from a base package or anything. I am planning to re-write them using Moose and I was hoping that rather than having to edit the Perl script that runs the download each time a new package is added, there might be a way of finding packages that inherit from a base package, and then in a loop instantiate each and do the downloading, kinda like so:
#! /usr/bin/perl
use strict;
for my $pname (packages_that_inherit_from("My::Package")) {
my $package = $pname->new;
$package->download;
}
Is it, or something ilke it, possible?
TIA
Using Moose's Class::MOP underpinning you can find the subclasses assigned to each class (at that point in time).
From Class::MOP::Class docs:
$metaclass->subclasses
This returns a list of all subclasses for this class, even indirect subclasses.
$metaclass->direct_subclasses
This returns a list of immediate subclasses for this class, which does not include indirect subclasses.
So for example if we build these classes:
{
package Root;
use Moose;
use namespace::clean -except => 'meta';
sub baz { say 'Some root thingy' }
sub download { say "downloading from " . __PACKAGE__ }
}
{
package NodeA;
use Moose;
extends 'Root';
use namespace::clean -except => 'meta';
sub download { say "downloading from " . __PACKAGE__ }
}
{
package NodeA1;
use Moose;
extends 'NodeA';
use namespace::clean -except => 'meta';
sub download { say "downloading from " . __PACKAGE__ }
}
Then using your example as a basis we can do this:
for my $pname ( Root->new->meta->direct_subclasses ) {
my $package = $pname->new;
$package->download;
}
# => "downloading from NodeA"
So above runs NodeA->download. Changing above to meta->subclasses would also run the NodeA1->download.
/I3az/
Although you say you are moving to Moose, a non-Moose way is to put all the derived packages in the known subdirectory based on the base package name. You then load all of the modules
For instance, if your base package is Local::Downloader, all the derived packages start with Local::Downloader::Plugin or something similar. You then look for all modules in your #INC that much .../Local/Downloader/Plugin/.... Although it's not too hard to do yourself, something like Module::PluginFinder can do it for you too.
What you are asking for wont be possible because none of the packages you are looking to use will be loaded yet. Why not place all of the packages in a common directory, and then have your script open that directory, and for each file, require it, and then instantiate your objects.
There's no way to do this based on inheritance because the parent class doesn't even know if it has descendants, never mind how many it has or what their names are.
However, if you follow the common convention of using hierarchal namespaces and naming the descendants as Parent::Foo, Parent::Bar, etc., you can approximate this by using Module::Pluggable to load up everything under the Parent namespace:
use Module::Pluggable require => 1, search_path => ['Parent'];
my #descendants = plugins();
Since this is based on the namespaces, though, it will pull in Parent::Helper::ThatIsNotAChild, while missing Child::NotUnder::Parent::Namespace, so it's not entirely perfect.
You know how Moose automatically turns on strict and warnings during import? I want to extend that behavior by turning on autodie and use feature ':5.10' in my Moose classes.
I've tracked down where Moose does this, in Moose::Exporter, which assembles a custom import sub for Moose that calls strict->import and warnings->import for the calling class.
However, I can't figure out a way to extend this import method in a Moose-ish way.
How should I handle this?
http://www.friedo.com/bullwinkle.gif
My approach solves the problem backwards.
Why not use ToolSet to create a group of use statements that includes Moose, along with your additional pragmas?
The code should look something like:
# MagicMoose.pm
package MagicMoose;
use base 'ToolSet';
ToolSet->use_pragma( qw/feature :5.10/ ); # perl 5.10
ToolSet->use_pragma( qw/autodie/ );
# define exports from other modules
ToolSet->export(
'Moose' => undef, # get the defaults
);
1; # modules must return true
I haven't tested this. Frankly, I just found ToolSet a few days ago, and haven't had a chance to try it out yet. FWIW, the reviews are positive.
Since there are many ways a module might export its functions into the use-ing namespace, you may need to do some code digging in order to implement each desired library. What you're asking for isn't anything specific to Moose, so you can write your or your company's own best practices module which will set up a group of standards for you to work with, e.g.
use OurCompany::BestPractices::V1;
with
package OurCompany::BestPractices::V1;
use strict;
use warnings;
use feature (':5.10');
require Fatal;
require Moose;
# Required for straight implementation of autodie code
our #ISA;
push #ISA, qw(
Fatal
);
sub import {
my $caller = caller;
strict->import;
warnings->import;
feature->import( ':5.10' );
Moose->import ({into => $caller});
#autodie implementation copied from autodie source
splice(#_,1,0,Fatal::LEXICAL_TAG);
goto &Fatal::import;
}
1;
Autodie makes things a little more complicated since it relies on finding the use-er's package from caller() and uses the goto, but you may be able to find a better way with more testing. The more you implement, the more complicated this library might be, but it might be of high enough value for you to have the one-off solution that you can use within all you or your company's code.
Moose::Exporter will allow you to define a custom import method for a sugar class you're using. MooseX::POE used a version of this for years, but does so in what I consider a "hackish" fashion. Looking at the documentation for Moose::Exporter the following should be roughly what you're asking for
package Modern::Moose;
use Moose ();
use Moose::Exporter;
my ($import) = Moose::Exporter->build_import_methods(
also => 'Moose',
install => [qw(unimport init_meta)],
);
sub import { # borrowing from mortiz's answer for feature/mro
feature->import( ':5.10' );
mro::set_mro( scalar caller(), 'c3' );
goto &$import;
}
This can then be used like so
package MyApp;
use Modern::Moose;
has greeting => (is => 'ro', default => 'Hello');
sub run { say $_[0]->greeting } # 5.10 is enabled
You have to define a sub called import in your package, and import all the other stuff there.
An example from Modern::Perl (another policy module you might look at):
use 5.010_000;
use strict;
use warnings;
use mro ();
use feature ();
sub import {
warnings->import();
strict->import();
feature->import( ':5.10' );
mro::set_mro( scalar caller(), 'c3' );
}
Update: Sorry, didn't read the question carefully enough.
A good way to extend an existing import method is to write your own in a new package, and call Moose's import method from there. You can do that by subclassing, maybe you can even use Moose yourself for that ;-)