The Moose is Flying, Part Two

Randal Schwartz

Tuesday, October 23rd, 2007

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Last month, I introduced the Moose object system by rewriting the sample code found in the perlboot man page to use Moose. Let’s continue the discussion and look at some of the features I didn’t cover last time.

Animal House

The role Animal included attributes name and color, and actions speak and eat. You can add a birthdate attribute to the Animal with:

 has ’born’ => (is => ’ro’); 

Since a birthdate isn’t mutable, it’s marked as read-only. By default, this attribute accepts any scalar, so these definitions are all equally valid:

 my $horse = Horse->new(born => ’yesterday’, name => ’Newbie’); my $cow = Cow->new(born => ’spring of 82’, name => ’Bessie’); my $mouse = Mouse->new(name => ’Minnie’, born => ’3/14/1929’); my $racehorse = RaceHorse->new(name => ’Slew’, born => [3, 5, 59]); 

You can use the Moose type system to narrow the permissible type of the birthdate attribute:

 require DateTime; has ’born’ => (is => ’ro’, isa => ’DateTime’); 

The isa parameter here declares that the born parameter must be a DateTime object, or at least something that responds true to UNIVERSAL::isa($thing,"DateTime"). Now, a run-time error occurs if you try to put anything in the born attribute other than a DateTime. Hence…

 my $horse = Horse->new(born => ’yesterday’, name => ’Newbie’); 

… fails, but this succeeds:

 my $horse = Horse->new(born => DateTime->now, name => ’Newbie’); 

The DateTime string for isa refers here to the…

1. Helpful Tools for Software Developers
2. The Github Hall of Fame
3. Book'em, Github.
4. This Week on Github: Stupid Ruby Tricks
5. A Veritable Scatter Shot!