The documentation for the lazy_build feature in Moose has this to say:
Note that use of this feature is strongly discouraged. Some documentation used to encourage use of this feature as a best practice, but we have changed our minds.
However, it does not explain what the reasoning for this is, and either my google-fu is terrible or there is no public explanation for why this is discouraged.
What's the problem with lazy_build that makes it discouraged today?
This is in Moose::Manual::BestPractices:
Avoid lazy_build
As described above, you rarely actually need a clearer or a predicate. lazy_build adds both to your public API, which exposes you to use cases that you must now test for. It's much better to avoid adding them until you really need them - use explicit lazy and builder options instead.
So what it's saying is that instead of using the property:
has attribute => (
...,
lazy_build => 1, # creates a builder called _build_attribute
);
You should instead be more explicit:
has attribute => (
...,
lazy => 1,
builder => '_build_attribute',
);
As that doesn't implicitly add clearer and predicate methods.
Related
I have a
package Test;
use Moose;
has 'attr' => ( is => 'rw', isa => 'Str' );
Inside a method I'd like to apply a s/pattern/string/g on the attribute. For reasons documented in Moose (basically to properly support polymorphism) I do not want to access the $self->{attr} directly, so a simple:
$self->{attr} =~ s/pattern/string/g;
is not an option. How can I do this efficiently in speed and little but clear code with Moose?
Options I came up with are:
1) Use a temporary variable, and the usual getter/setter method:
my $dummy = $self->attr;
$dummy =~ s/pattern/string/g;
$self->attr($dummy);
2) Using the attr getter/setter on the left hand side:
$self->attr($dummy) =~ s/pattern/string/g;
But this obviously throws an error
Can't modify non-lvalue subroutine call at Test.pm
line 58, line 29
Is there a way to use Moose accessors as lvalue subs?
3) Use the String traits
Redefine the attribute:
has 'attr' => ( is => 'rw', isa => 'Str', traits => ['String'],
handles => { replace_attr => 'replace'} );
Then in the method use:
$self->replace_attr('pattern', 'string');
However the docs explicitly say, there's no way to specify the /g flag.
Any elegant, simple, somewhat efficient method available out of the box?
I have used this approach in the past and I think it seems suitable to me for general use in terms of efficiency and cleanliness. It also works with the /g modifier.
$self->attr( $self->attr =~ s/pattern/string/gr );
I suspect that under the hood this is the same as your first example with the temporary variable, it is just hidden from us.
Please note that the to use the /r modifier, which returns the result of the substitution without modifying the original, requires Perl 5.14+.
My Option (2) and this question provide the idea to use MooseX::LvalueAttributes:
package Test;
use Moose;
use MooseX::LvalueAttribute 'lvalue';
has 'attr' => ( is => 'rw', isa => 'Str', traits => [lvalue] );
This allows the straightforward syntax:
$self->attr($dummy) =~ s/pattern/string/g;
Internally this uses Variable::Magic and the perlsub lvalue feature, so there is a performance overhead to this approach which affects every access to the 'traited' attribute, not just the ones where it's used as a left hand side. Thanks to LeoNerd and ikegami for their correcting comments on my earlier statements.
Therefore, and confirmed by the module's documentation, Moose's type checking still works and triggers are fired.
I want to be able to instantiate a Moose based object add to it until I serialize it and then I want to make it unchangeable. How can/should I go about doing this?
I would make two classes and a common Role:
package Thing
use Moose::Role;
has some_attrib => (isa => 'AnotherThing');
### Behaviour (the important stuff) goes here
package ImmutableThing;
use Moose;
with 'Thing';
has +some_attrib => (is => 'ro');
sub finalize { shift }
package MutableThing
use Moose;
with 'Thing';
has +some_attrib => (is => 'rw');
sub finalize {
my $self = shift;
Thing->new({some_attrib => $self->some_attrib});
}
I'm not sure that having mutable and immutable forms of the same class is necessarily a good idea though. I tend to try and think about build time and operation time as two distinct phases with different interfaces.
I would be more inclined to write a Parameter Collector (I've capitalised it like it's a pattern, but I've not seen it in the literature) that has an interface optimised to gathering the info needed to create a Thing, and the Thing Itself, which is the object that's used by the rest of the program.
I don't know of (and can't easily find) any modules to do this on CPAN which is surprising but explains why you are asking :-)
A "before" modifier over all your attributes is the obvious way to go about it. I'm sure there's a suitable meta-programming way to get a list of all attribute accessors and apply the modifier, but I'd be tempted to explicitly list them all with a big comment.
Have you considered whether you have one class or two here (Thingy, LockedThingy)? Two classes would let you encapsulate the meta cleverness if you're that way inclined.
My Moo based class has both lazy & non-lazy attributes which have both default and coerce subs. If I don't initialize the attributes I'm finding that both default and coerce subs are called for the normal attribute, but only default is called for the lazy attribute. That seems inconsistent. Here's sample code:
package Foo;
use Moo;
has nrml => ( is => 'ro',
default => sub { print "nrml default\n" },
coerce => sub { print "nrml coerce\n" }
);
has lazy => ( is => 'ro',
lazy => 1,
default => sub { print "lazy default\n" },
coerce => sub { print "lazy coerce\n" }
);
my $q = Foo->new( );
$q->lazy;
The output is:
nrml default
nrml coerce
lazy default
I only expect coerce to run if I provide a value in the constructor. More importantly I expect the same sequence of execution (either default or default and coerce) from both lazy and normal attributes.
So, are my expectations off, is this a bug, or what? Thanks!
Current status: fix shipped in 009014
One of those two is a bug.
In fact, thinking about it, one could argue either way about whether coercions -should- be fired on defaults but since Moose does do so, and since coercions are structural (unlike type checks, which are often used for assertion-like things and should always pass except in the presence of a bug), I think it falls that way.
... in fact, the problem is that Method::Generate::Accessor when it fires _use_default always wraps it in _generate_simple_set, when it's _generate_set that provides the isa+coerce+trigger wrapping - and I'm fairly sure that Moose fires all three when it's applying a default, so we need to too.
It's not an entirely trivial fix to make though, because I didn't parameterise _generate_set to take a value indicating how to generate the value to set. I'll try and sort it out tomorrow since I'm planning to cut a release then.
If you want support for Moo from the developers, please contact bugs-Moo#rt.cpan.org or join #web-simple on irc.perl.org - it's sheer luck that somebody on the IRC channel saw this question and asked about it :)
That would qualify as a bug to me. Either the value from default is expected to be of the right type, or it's not. Having and enforcing the expectation only half of the time makes no sense.
This may turn out to be an embarrassingly stupid question, but better than potentially creating embarrassingly stupid code. :-) This is an OO design question, really.
Let's say I have an object class 'Foos' that represents a set of dynamic configuration elements, which are obtained by querying a command on disk, 'mycrazyfoos -getconfig'. Let's say that there are two categories of behavior that I want 'Foos' objects to have:
Existing ones: one is, query ones that exist in the command output I just mentioned (/usr/bin/mycrazyfoos -getconfig`. Make modifications to existing ones via shelling out commands.
Create new ones that don't exist; new 'crazyfoos', using a complex set of /usr/bin/mycrazyfoos commands and parameters. Here I'm not really just querying, but actually running a bunch of system() commands. Affecting changes.
Here's my class structure:
Foos.pm
package Foos, which has a new($hashref->{name => 'myfooname',) constructor that takes a 'crazyfoo NAME' and then queries the existence of that NAME to see if it already exists (by shelling out and running the mycrazyfoos command above). If that crazyfoo already exists, return a Foos::Existing object. Any changes to this object requires shelling out, running commands and getting confirmation that everything ran okay.
If this is the way to go, then the new() constructor needs to have a test to see which subclass constructor to use (if that even makes sense in this context). Here are the subclasses:
Foos/Existing.pm
As mentioned above, this is for when a Foos object already exists.
Foos/Pending.pm
This is an object that will be created if, in the above, the 'crazyfoo NAME' doesn't actually exist. In this case, the new() constructor above will be checked for additional parameters, and it will go ahead and, when called using ->create() shell out using system() and create a new object... possibly returning an 'Existing' one...
OR
As I type this out, I am realizing it is perhaps it's better to have a single:
(an alternative arrangement)
Foos class, that has a
->new() that takes just a name
->create() that takes additional creation parameters
->delete(), ->change() and other params that affect ones that exist; that will have to just be checked dynamically.
So here we are, two main directions to go with this. I'm curious which would be the more intelligent way to go.
In general it's a mistake (design-wise, not syntax-wise) for the new method to return anything but a new object. If you want to sometimes return an existing object, call that method something else, e.g. new_from_cache().
I also find it odd that you're splitting up this functionality (constructing a new object, and returning an existing one) not just into separate namespaces, but also different objects. So in general, you're closer with your second approach, but you can still have the main constructor (new) handle a variety of arguments:
package Foos;
use strict;
use warnings;
sub new
{
my ($class, %args) = #_;
if ($args{name})
{
# handle the name => value option
}
if ($args{some_other_option})
{
# ...
}
my $this = {
# fill in any fields you need...
};
return bless $this, $class;
}
sub new_from_cache
{
my ($class, %args) = #_;
# check if the object already exists...
# if not, create a new object
return $class->new(%args);
}
Note: I don't want to complicate things while you're still learning, but you may also want to look at Moose, which takes care of a lot of the gory details of construction for you, and the definition of attributes and their accessors.
It is generally speaking a bad idea for a superclass to know about its subclasses, a principle which extends to construction.[1] If you need to decide at runtime what kind of object to create (and you do), create a fourth class to have just that job. This is one kind of "factory".
Having said that in answer to your nominal question, your problem as described does not seem to call for subclassing. In particular, you apparently are going to be treating the different classes of Foos differently depending on which concrete class they belong to. All you're really asking for is a unified way to instantiate two separate classes of objects.
So how's this suggestion[3]: Make Foos::Exists and Foos::Pending two separate and unrelated classes and provide (in Foos) a method that returns the appropriate one. Don't call it new; you're not making a new Foos.
If you want to unify the interfaces so that clients don't have to know which kind they're talking about, then we can talk subclassing (or better yet, delegation to a lazily-created and -updated Foos::Handle).
[1]: Explaining why this is true is a subject hefty enough for a book[2], but the short answer is that it creates a dependency cycle between the subclass (which depends on its superclass by definition) and the superclass (which is being made to depend on its subclass by a poor design decision).
[2]: Lakos, John. (1996). Large-scale C++ Software Design. Addison-Wesley.
[3]: Not a recommendation, since I can't get a good enough handle on your requirements to be sure I'm not shooting fish in a dark ocean.
It is also a factory pattern (bad in Perl) if the object's constructor will return an instance blessed into more than one package.
I would create something like this. If the names exists than is_created is set to 1, otherwise it is set to 0.. I would merge the ::Pending, and ::Existing together, and if the object isn't created just put that into the default for the _object, the check happens lazily. Also, Foo->delete() and Foo->change() will defer to the instance in _object.
package Foo;
use Moose;
has 'name' => ( is => 'ro', isa => 'Str', required => 1 );
has 'is_created' => (
is => 'ro'
, isa => 'Bool'
, init_arg => undef
, default => sub {
stuff_if_exists ? 1 : 0
}
);
has '_object' => (
isa => 'Object'
, is => 'ro'
, lazy => 1
, init_arg => undef
, default => sub {
my $self = shift;
$self->is_created
? Foo->new
: Bar->new
}
, handles => [qw/delete change/]
);
Interesting answers! I am digesting it as I try out different things in code.
Well, I have another variation of the same question -- the same question, mind you, just a different problem to the same class:subclass creation issue!
This time:
This code is an interface to a command line that has a number of different complex options. I told you about /usr/bin/mycrazyfoos before, right? Well, what if I told you that that binary changes based on versions, and sometimes it completely changes its underlying options. And that this class we're writing, it has to be able to account for all of these things. The goal (or perhaps idea) is to do: (perhaps called FROM the Foos class we were discussing above):
Foos::Commandline, which has as subclasses different versions of the underlying '/usr/bin/mycrazyfoos' command.
Example:
my $fcommandobj = new Foos::Commandline;
my #raw_output_list = $fcommandobj->getlist();
my $result_dance = $fcommandobj->dance();
where 'getlist' and 'dance' are version-dependent. I thought about doing this:
package Foos::Commandline;
new (
#Figure out some clever way to decide what version user has
# (automagically)
# And call appropriate subclass? Wait, you all are telling me this is bad OO:
# if v1.0.1 (new Foos::Commandline::v1.0.1.....
# else if v1.2 (new Foos::Commandline::v1.2....
#etc
}
then
package Foos::Commandline::v1.0.1;
sub getlist ( eval... system ("/usr/bin/mycrazyfoos", "-getlistbaby"
# etc etc
and (different .pm files, in subdir of Foos/Commandline)
package Foos::Commandline::v1.2;
sub getlist ( eval... system ("/usr/bin/mycrazyfoos", "-getlistohyeahrightheh"
#etc
Make sense? I expressed in code what I'd like to do, but it just doesn't feel right, particularly in light of what was discussed in the above responses. What DOES feel right is that there should be a generic interface / superclass to Commandline... and that different versions should be able to override it. Right? Would appreciate a suggestion or two on that. Gracias.
Perl has OOP features, but they are somewhat rarely used. How do you create and use Perl objects with methods and properties?
You should definitely take a look at Moose.
package Point;
use Moose; # automatically turns on strict and warnings
has 'x' => (is => 'rw', isa => 'Int');
has 'y' => (is => 'rw', isa => 'Int');
sub clear {
my $self = shift;
$self->x(0);
$self->y(0);
}
Moose gives you (among other things) a constructor, accessor methods, and type checking for free!
So in your code you can:
my $p = Point->new({x=>10 , y=>20}); # Free constructor
$p->x(15); # Free setter
print $p->x(); # Free getter
$p->clear();
$p->x(15.5); # FAILS! Free type check.
A good starting point is Moose::Manual and Moose::Cookbook
If you just need the basic stuff you can also use Mouse which is not as complete, but without most of the compile time penalty.
Moose, definitely.
package Person;
use Moose;
has age => ( isa => Int, is => 'rw');
has name => ( isa => Str, is => 'rw');
1;
Immediately, you have for free a new() method, and accessor methods for the attributes you just defined with 'has'. So, you can say:
my $person = Person->new();
$person->age(34);
$person->name('Mike');
print $person->name, "\n";
and so on. Not only that, but your accessor methods come type-checked for free (and you can define your own types as well as the standard ones). Plus you get 'extends' for subclassing, 'with' for roles/traits, and all manner of other great stuff that allows you to get on with the real job of writing good robust maintainable OO code.
TMTOWTDI, but this one works.
Currently I use Object::InsideOut whenever I want objects, its quite nice and will give you a lot of features over standard blessed hash objects. Having said that, if I was starting a new project I would seriously look at Moose.
While it is good to read the official PERL documentation, I would NOT recommend trying to role your own object framework, or building objects using hashes, its far to tempting to take the easy road and "peak" directly into the objects "private" variables completely breaking encapsulation, this will come back to bite you when you want to refactor the object.
Perl objects are NOT just blessed hashes. They are blessed REFERENCES. They can be (and most often are) blessed hash references, but they could just as easily be blessed scalar or array references.
The official tutorial on the CPAN site is good.
There's also a good article called Camel POOP at CodeProject.
I highly recommend taking a look at Moose if you want to do OO in Perl. However, it's not very useful if you don't understand what OO in Perl means. To better understand how Perl OO works under the hood, I wrote an overview on my blog: http://augustinalareina.wordpress.com/2010/06/06/an-introduction-to-object-oriented-perl/
From a data structure point of view, an Object is reference with a few extra features. The interpreter knows to treat these special references as Objects because they have been "blessed" with the keyword "bless". Blessed references contain a flag indicating they are an Object. Essentially this means you can define and call methods on them.
For instance if you created a basic hashref, this wouldn't work:
$hashref->foo();
But if you create a blessed hashref (aka an Object) this does work:
$blessed_hashref->foo();
Moose is an excellent module for OOP in Perl because it creates an enforceable OO layer AND automagically handles accessor methods so you don't have to define a bunch of getters and setters. If you're interested in using Devel::Peak to see how the Perl interpreter stores objects, follow the link to the blog entry I posted above.
On one foot, each class is a package; you establish (multiple, if desired) inheritance by setting the package variable #ISA (preferably at compile time); you create an object from an existing piece of data (often, but not always, an anonymous hash used to store instance variables) with bless(REFERENCE [, CLASSNAME]); you call object methods like $obj->methodname(#ARGS) and class methods like "CLASSNAME"->methodname(#ARGS).
Multiple inheritance method resolution order can be altered using mro.
Because this is somewhat minimalistic and doesn't force encapsulation, there are many different modules that provide more or different functionality.