class variables in Moose

Moose attribute didn't allow the reference rather than a CODE reference as a default value. That prevents the objects shared a attribute when initializing. Actually this is a good design. But How should I do when needing a shared attribute between objects? I may refer this as the concept of class variable.

The state variable in CODE reference may be competent with this role.

A Counter as an example here. We define a Counter class here.

	use Modern::Perl;
	use MooseX::Declare;
	class Counter {
	has value => (
	is => "rw",
	isa => "Num",
	traits => ['Counter'],
	default => 0,
	handles => {
	inc => "inc",
	},
	);
	};

view raw Counter.pm hosted with ❤ by GitHub

A counter has a attribute "value" and a method "inc" to increment it.

Then We define a Person class.

	use Modern::Perl;
	use MooseX::Declare;
	use Counter;
	class Person {
	has name => (
	is => "ro",
	isa => "Str",
	default => "nobody",
	required => 1,
	);
	has count => (
	is => "rw",
	isa => "Counter",
	default => sub {
	state $count = Counter->new;
	$count;
	},
	);
	sub BUILD {
	my ($self) = shift;
	$self->count->inc;
	}
	};

view raw Person.pm hosted with ❤ by GitHub

The Person class has a attribute "name" and a shared attribute "count". The Counter is a state variable return from a anonymous CODE reference. That's the trick.

The BUILD hook increment the counter everytime a new Person is constructed.

Then we examinate the shared variable.

	#!/usr/bin/env perl
	#-*- mode: cperl -*-
	use Modern::Perl;
	use Person;
	my $shelling = Person->new(name => "shelling");
	my $count = $shelling->count;
	say $count->value; #=> 1
	my $sherry = Person->new(name => "sherry");
	say $count->value; #=> 2
	my $appollo = Person->new(name => "appollo");
	say $count->value; #=> 3

view raw class-variables.pl hosted with ❤ by GitHub

A global variable in package name may still work. But this way uses 'has' syntax sugar. :p

If you use builder to replace the default option. the subroutine used by the builder is just the class variable accessor from Class name, returning value as $person->count().

Mysterious Timestamp

時間轉換大概是最惱人的「小」問題之一，每個語言的標準處理方式不盡相同。Perl 的 localtime() 是月份從 0 開始到 11 結束，Ruby 的 DateTime 是月份是從 1 開始的，不過 Perl 的 DateTime 月份也從 1 開始 (LOL)。背誦這些東西大概會發瘋。

不過 Epoch 沒有這個問題，任何語言上的 Epoch 應該都是一致的，有的只是時差問題，使用 Epoch 來初始時間物件大概是避開這問題的好方法之一，特別在不同語言間交換時間資料時，用 Epoch 交換後怎麼 format 都不會出問題。所以有些時間模組大概會像 Perl 的 DateTime 一樣實作 from_epoch() 這種 constructor。

就這點設計而言，Badger::Timestamp 大概是最好用的時間模組，其 constructor 支援 DateTime 模組的方式初始化外，還可以餵食 Epoch 和 ISO 格式的時間字串。一個簡單的 Epoch/ISO converter 只要八行。

	#!/usr/bin/env perl
	use 5.010;
	use Badger::Timestamp;
	my $ts = Badger::Timestamp->new($ARGV[0]);
	say $ts->timestamp;
	say $ts->epoch_time;

view raw timestamp.pl hosted with ❤ by GitHub

什麼，你問說這八行程式有什麼用，有些跑了好幾年的 unix 系統上有著 crontab 執行著需要 timestamp 來分辨執行區段的分析器，有些要當天的零分零秒，有些要隔天的零分零秒。壞掉時有人會要你一定要「人工」處理，這八行就就是這樣來的。

附帶一提， Javascript 的 Date.UTC 和 Perl 的 localtime() 一樣，都是月份從 0 開始。

為什麼Javascript的月份是從0開始的 (-_-) w3schools.com/jsref/jsref_ut…

— Alec Chen (@alec929) February 28, 2012

和 Badger::Timestamp 很像的 moment.js這時候就很好用了，其 constructor 用陣列初始化時雖然還是沒有改用 1 作為月份起始著實也讓我驚訝了一下。好險，這 constructor 支援用 Epoch 初始化(不過承襲 javascript 的慣例，是 millisecond)，還可以用 Date.parse() 支援的字串初始化。這兩招可以避開月份的問題。