Agent skill

acc-create-null-object

Generates Null Object pattern for PHP 8.5. Creates safe default implementations eliminating null checks. Includes unit tests.

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npx add-skill https://github.com/majiayu000/claude-skill-registry/tree/main/skills/data/acc-create-null-object

SKILL.md

Null Object Pattern Generator

Creates Null Object pattern infrastructure for eliminating null checks.

When to Use

Scenario Example
Optional dependencies Logger, Cache, Notifier
Missing entities Customer, User, Product
Feature toggles Disabled feature returns null object
Default implementations No-op defaults

Component Characteristics

Interface

  • Defines expected behavior
  • Shared by real and null implementations
  • Enables polymorphic usage

Null Object

  • Implements interface with no-op behavior
  • Returns neutral values
  • Never throws on calls

Benefits

  • Eliminates null checks
  • Follows Liskov Substitution
  • Simplifies client code

Generation Process

Step 1: Generate Interface

Path: src/Domain/{BoundedContext}/

  1. {Name}Interface.php — Interface with isNull() method

Step 2: Generate Null Object

Path: src/Domain/{BoundedContext}/

  1. Null{Name}.php — Null implementation returning neutral values

Step 3: Generate Real Implementation

Path: src/Domain/{BoundedContext}/

  1. {Name}.php — Real implementation with business logic

Step 4: Generate Tests

  1. Null{Name}Test.php — Null object behavior tests

File Placement

Component Path
Interface src/Domain/{BoundedContext}/
Null Object src/Domain/{BoundedContext}/
Real Implementation src/Domain/{BoundedContext}/
Unit Tests tests/Unit/Domain/{BoundedContext}/

Naming Conventions

Component Pattern Example
Interface {Name}Interface CustomerInterface
Real Implementation {Name} Customer
Null Object Null{Name} NullCustomer
Test {ClassName}Test NullCustomerTest

Neutral Return Values

Type Neutral Value
string '' (empty string)
int 0
float 0.0
bool false
array []
void No return
Object Empty/default instance
Collection Empty collection

Quick Template Reference

Interface

php
interface {Name}Interface
{
    public function {method1}(): {returnType1};
    public function {method2}({params}): {returnType2};
    public function isNull(): bool;
}

Null Object

php
final readonly class Null{Name} implements {Name}Interface
{
    public function {method1}(): {returnType1}
    {
        return {neutralValue1};
    }

    public function isNull(): bool
    {
        return true;
    }
}

Real Implementation

php
final readonly class {Name} implements {Name}Interface
{
    public function __construct({properties}) {}

    public function {method1}(): {returnType1}
    {
        return {realImplementation};
    }

    public function isNull(): bool
    {
        return false;
    }
}

Usage Example

Repository Returning Null Object

php
final readonly class DoctrineCustomerRepository implements CustomerRepositoryInterface
{
    public function findById(CustomerId $id): CustomerInterface
    {
        $row = $this->connection->fetchAssociative(
            'SELECT * FROM customers WHERE id = :id',
            ['id' => $id->toString()]
        );

        if ($row === false) {
            return new NullCustomer();
        }

        return $this->hydrate($row);
    }
}

Client Code Without Null Checks

php
final readonly class CreateOrderUseCase
{
    public function execute(CreateOrderCommand $command): Order
    {
        $customer = $this->customers->findById($command->customerId);

        // No null check needed - NullCustomer returns 0.0
        $discount = $customer->getDiscount();

        $order = Order::create(
            customerId: $customer->id(),
            items: $command->items,
            discount: $discount
        );

        return $order;
    }
}

Anti-patterns to Avoid

Anti-pattern Problem Solution
Throwing in Null Unexpected exceptions Return neutral values
No isNull() Can't detect null object Add isNull method
Different Interface LSP violation Same interface as real
Side Effects Unexpected behavior Pure no-op methods
Complex Logic Not a null object Keep simple and neutral

References

For complete PHP templates and examples, see:

  • references/templates.md — Interface, NullObject, Real Implementation, NullLogger, NullCache templates
  • references/examples.md — NullCustomer, NullNotifier, Repository integration examples and tests

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