Saturday, January 1, 2011

Series Perl

Table of contents

Namespaces

One package should never mess with the namespace of another package unless it has been explicitly told to do so. Thus, never define methods in another script and require it in. Always wrap your library in a package and use it. This way your namespaces will remain cleanly separated: 
package Sophie;
sub say_hello {
    print "Hi World!";
}

package Clara;
use Sophie;           # loads the package but does
                      #  NOT import any methods
say_hello();          # blows up
Sophie->say_hello();  # correct usage
Sophie::say_hello();  # works, but not for inherited methods

Rooting a namespace

When you use an unloaded package Some::Package, Perl looks for a file Some/Package.pm in the current directory. If this file doesn't exist, it looks for other namespace roots (like c:/perl/lib) in the global @INC array.
It is a good idea to save your application packages to a directory like lib and add that directory to the list of namespace roots using use lib 'my/root/path': 
use lib 'lib';      # Add the sub-directory 'lib'
                    # to the namespace root @INC 
use Some::Package;  # Walks through @INC to find 
                    # the package file

Exporting symbols

There are rare occasions when you do want to export methods or variable names into the calling package. I only do this occasionally for static helper methods I need very, very often. In order to export symbols, inherit from the Exporter class and fill the @EXPORT array with the symbols you'd like to export: 
package Util;
use base 'Exporter';
our @EXPORT = ('foo', 'bar');

sub foo {
    print "foo!";
}
sub bar {
    print "bar!";
}

package Amy;
use Util;  # imports symbols in @EXPORT
foo();     # works fine
bar();     # works fine
Try not to pollute another package's namespace unless you have a very good reason for doing so! Most packages on CPAN explicitly state which symbols get exported with their use, if any.
It might be a good idea to leave it up to the requiring package to decide which symbols get exported into its namespace. In that case you simply use the @EXPORT_OK array instead or @EXPORT. 
package Util;
use base 'Exporter';
our @EXPORT_OK = ('foo', 'bar');

sub foo {
    print "foo!";
}
sub bar {
    print "bar!";
}

package Amy;
use Util 'foo';  # only import foo()
foo();           # works fine
bar();           # blows up

Instant data structures

Use { } to create anonymous hash references. Use [ ] to create anonymous array references. Combine these constructs to create more complex data structures such as lists of hashes: 
my @students = ( { name         => 'Clara',
                   registration => 10405,
                   grades       => [ 2, 3, 2 ] },
                 { name         => 'Amy',
                   registration => 47200,
                   grades       => [ 1, 3, 1 ] },
                 { name         => 'Deborah',
                   registration => 12022,
                   grades       => [ 4, 4, 4 ] } );
Use -> to dereference the structure and get to the values: 
# print out names of all students
foreach my $student (@students) {
    print $student->{name} . "\n";
}

# print out Clara's second grade
print $students[0]->{grades}->[1];

# delete Clara's registration code
delete $students[0]->{registration};

Classes and objects

Packages are classes. Objects are usually hash references bless-ed with a class name. Attributes are key/value pairs in that hash.

Constructors

Constructors are static methods that happen to return objects: 
package Student;
sub new {
my($class, $name) = @_;        # Class name is in the first parameter
my $self = { name => $name };  # Anonymous hash reference holds
                               # instance attributes  
bless($self, $class);          # Say: $self is a $class
return $self;
}

package main;
use Student;
my $amy = Student->new('Amy');
print $amy->{name}; # Accessing an attribute
Instead of Student->new('Amy') you may also write new Student('Amy'). Note however that the Perl parser relies on some funky heuristics to guess your true intentions and sometimes guesses wrong.

Multiple constructors

Because the new keyword is in no way magical in Perl, you may have as many constructor methods as you like and give them any name you like. For instance, you might want different constructors depending on whether you'd like to form an object from an existing record in a database, or create a new instance from scratch: 
my $amy   = Student->existing('Amy');
my $clara = Student->create();
As a constructor explicitly returns the constructed object, $self isn't magical either. You might, for example, retrieve $self from a static cache of already constructed objects: 
package Coke;
my %CACHE;

sub new {
 my($class, $type) = @_;
 return $CACHE{$type} if $CACHE{$type};   # Use cache copy if possible
 my $self = $class->from_db($type);       # Fetch it from the database
 $CACHE{$type} = $self;                   # Cache the fetched object
 return $self;
}

sub from_db {
my($class, $type) = @_;
my $self = ...         # Fetch data from the database 
bless($self, $class);  # Make $self an instance of $class
return $self;
}

package main;
use Coke;

my $foo = Coke->new('Lemon');    # Fetches from the database
my $bar = Coke->new('Vanilla');  # Fetches from the database
my $baz = Coke->new('Lemon');    # Uses cached copy
For the sake of completeness I should note that the references in %CACHE will keep cached objects alive even if all their other instances cease to exist. Thus, should your cached objects have destructor methods defined, they won't be called until the program gets terminated.

Instance Methods

Instance methods get a reference to the called object in their first parameter: 
package Student;

sub work {
    my($self) = @_;
    print "$self is working\n";
}
sub sleep {
    my($self) = @_;
    print "$self is sleeping\n";
}

package main;
use Student;

my $amy = Student->new('Amy');
$amy->work();
$amy->sleep();
The reference to itself (this in Java) is never implicit in Perl: 
sub work {
    my($self) = @_;
    sleep();         # Don't do this
    $self->sleep();  # Correct usage
}

Static methods

Static methods get the name of the calling class in their first parameter. Constructors are simply static methods: 
package Student;

sub new {
    my($class, $name) = @_;
    # ...
}
sub list_all {
    my($class) = @_;
    # ...
}

package main;
use Student;
Student->list_all();
Instance methods can call static methods using $self->static_method(): 
sub work {
    my($self) = @_;
    $self->list_all();
}

Inheritance

Inheritance works through use base 'Base::Class': 
package Student::Busy;
use base 'Student';

sub night_shift {
    my($self) = @_;
    $self->work();
}

sub sleep {  # Overwrite method from parent class
    my($self) = @_;
    $self->night_shift();
}
All classes automatically inherit some basic functionality like isa and can from the UNIVERSAL class. Also if you feel the urge to shoot yourself in the foot by using multiple inheritance, Perl is not going to stop you.

Strict instance attributes

As our vanilla object is a simple hash reference, you may use any attribute name and Perl won't complain: 
use Student;
my $amy = Student->new('Amy');
$amy->{gobbledegook} = 'some value';  # works
Often you'd rather like to give a list of attributes that are allowed, and have Perl exit with an error if someone uses an unknown attribute. You do this using the fields pragma: 
package Student;

use fields 'name',
           'registration',
           'grades';

sub new {
my($class, $name) = @_;
$self = fields::new($class);  # returns an empty "strict" object
$self->{name} = $name;        # attributes get accessed as usual
return $self;                 # $self is already blessed
}

package main;
use Student;

my $clara = Student->new('Clara');
$clara->{name} = 'WonderClara';  # works
$clara->{gobbledegook} = 'foo';  # blows up

A note on the uniform access principle

Some of you might turn up their noses at the way I access instance attributes in my examples. Writing $clara->{name} is fine as long as I only need to return a stored value. However, should my Student package change in a way that returning a {name} requires some kind of computation (like combining it from a {first_name} and {last_name}), what should I do? Obviously changing the public interface of my package and replace all occurences of $clara->{name} with $clara->get_name() is no acceptable option.
Basically you are left with two options:
  • It is possible to retrospectively tie the scalar in $clara->{name} to a class that does the required computation whenever someone gets or sets the attribute. I find this process to be somewhat laborious in vanilla Perl, but take a look at the perltie page in the Perl documentation to get your own picture.
  • Use accessor methods (aka getters and setters) exclusively and outlaw direct attribure access in your software project. I personally prefer this alternative because it makes for prettier code and gives me control over which attributes are visible to other classes. CPAN carries various module which automate the creation of accessor methods. I will show you how to roll your own accessor generator in Extending the language.

Imports

Because the packages you use get imported at compile-time you can completely change the playing field before the interpreter even gets to look at the rest of your script. Thus imports can be extremely powerful.

Import parameters

You can hand over parameters to any package you use: 
package Student;
use Some::Package 'param1', 'param2';
Whenever you use a package, the static method import is called in that package with all parameters you might have given: 
package Some::Package;
sub import {
    my($class, @params) = @_;
}

Who's calling?

The caller() function lets you (among other things) find out what class was calling the current method: 
package Some::Package;
sub import {
    my($class, @params) = @_;
    print "Look, " . caller() . " is trying to import me!";
}

Extending the language

Let's combine what we know and write a simple package members that sets fields for the calling package, and while it's at it produces convenient accessor methods for these fields: 
package members;

sub import {
  my($class, @fields) = @_;
  return unless @fields;
  my $caller = caller();
 
  # Build the code we're going to eval for the caller
  # Do the fields call for the calling package
 
my $eval = "package $caller;\n" .
  "use fields qw( " . join(' ', @fields) . ");\n";

  # Generate convenient accessor methods
  foreach my $field (@fields) {
  $eval .= "sub $field : lvalue { \$_[0]->{$field} }\n";
}

  # Eval the code we prepared
  eval $eval;

  # $@ holds possible eval errors
  $@ and die "Error setting members for $caller: $@";
}

# In a nearby piece of code...

package Student;
use members 'name',
            'registration',
            'grades';

sub new {
    my($class, $name) = @_;
    $self = fields::new($class);
    $self->{name} = $name;
    return $self;
}

package main;
my $eliza = Student->new('Eliza');
print $eliza->name;            # Look Ma, no curly brackets!
                               # Same as $eliza->name()
$eliza->name = 'WonderEliza';  # Works because our accessors
                               #  are lvalue methods
print $eliza->name;            # Prints "WonderEliza"
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