state.md

State != Behavior

Ovid has discovered another "bug" (and by "bug" I mean documented feature ;) ). He has code that effectively does:

class MyClass {
    has my_method => ( default => 1);
}

and then is surprised when MyClass->new->my_method fails. In his case it was hidden by an inheritance hierarchy. This bites many people new to Moose, and there has been a suggestion to make has default to something. The problem with a default for has is that there are valid uses for not providing an accessor.

class Toy {
    has sound_chip => {        
        default  => sub { SoundChip->new() }
        handles  => [qw(beep)],
    );
}

Then you can say Toy->new->beep. We never want people to even know that a Toy uses a SoundChip object because it's entirely encapsulated. The only external exposure right now is someone could in theory say Toy->new(sound_chip => $geigerCounter). We can entirely encapsulate this with:

class Toy {
    has sound_chip => {        
        init_arg: undef, # disable new( sound_chip => Object )
        default  => sub { SoundChip->new() }
        handles  => [qw(beep)],
    );
}

A more extended example (and closer to stuff I've seen in production) uses Roles.

role TemperatureAdjustments { requires 'adjust_temperature' }

class Furnace {
    has thermostat => (
        does     => 'TemperatureAdjustments',
        handles  => 'TemperatureAdjustments',
        default => sub { StandardThermostat->new() }
    )
}

then later you say:

my $furnace = Furnace->new();
$furnace->adjust_temperature("+1F");

Once the widget is integrated into the Furnace as a thermostat there is no valid use case to access it directly. Providing a default for has means that this is no longer simple and elegant. But! you say, shouldn't we optimize for the most common behavior? Yes we should, and you'd be right to change the default for has if it worked the way Ovid assumed it does.

class MyClass {
    has my_method => ( default => 1);
}

This starts with entirely the wrong assumption at my_method. You're not declaring a method here, you are declaring an attribute. Attributes are state and state != behavior. Moose allows you to also define behavior via has. is, reader, writer, clearer, predicate, handles, and lazy_build all enable some kind of behavior in your class.

By making a default for is what you're doing is creating action at a distance and invisibly coupling the definition of behavior with the definition of state.