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Earlier we saw how default values can be set for a data object, sometimes you want to set a default value based on other properties. For example, you might want to set a full_name property based on a first_name and last_name property. You can do this by using a computed property: 
use Spatie\LaravelData\Attributes\Computed;

class SongData extends Data
{
    #[Computed]
    public string $full_name;

    public function __construct(
        public string $first_name,
        public string $last_name,
    ) {
        $this->full_name = "{$this->first_name} {$this->last_name}";
    }
}
You can now do the following: 
SongData::from(['first_name' => 'Ruben', 'last_name' => 'Van Assche']);
Please notice: the computed property won’t be reevaluated when its dependencies change. If you want to update a computed property, you’ll have to create a new object.

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Requirements for Computed Properties

Again there are a few conditions for this approach:
1

Use a sole property

You must always use a sole property, a property within the constructor definition won’t work
2

Cannot be set in payload

Computed properties cannot be defined in the payload, a CannotSetComputedValue will be thrown if this is the case
3

Silent mode available

If the ignore_exception_when_trying_to_set_computed_property_value configuration option is set to true, the computed property will be silently ignored when trying to set it in the payload and no CannotSetComputedValue exception will be thrown