Primitive Obsession

Also Known As

TL;DR

Explanation

Primitive obsession is using primitive types (string, int, bool, array) for concepts that deserve their own class — e.g. representing a phone number as a plain string, a money amount as a float, or a coordinate pair as two separate variables. Primitives have no domain validation, no domain-specific methods, and no documentation of their meaning. Introducing small value objects (PhoneNumber, Money, Coordinate) gives the type a name, centralises validation, and makes illegal states unrepresentable.

Common Misconception

Why It Matters

Common Mistakes

• Passing $email as a string everywhere instead of an Email value object that validates on construction.
• Using int for money — floating point errors and currency handling belong in a Money class.
• Not noticing when a cluster of primitives always travels together — that is a data clump and a value object candidate.
• Creating value objects without making them immutable — a mutable Email can change to invalid state after construction.

Code Examples

function registerUser(string $email, string $role, int $age): void {
    if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
        throw new \InvalidArgumentException('Bad email');
    }
    // $email, $role, $age scattered everywhere as raw strings/ints
}

readonly class Email {
    public function __construct(public string $value) {
        if (!filter_var($value, FILTER_VALIDATE_EMAIL)) {
            throw new \InvalidArgumentException("Invalid email: $value");
        }
    }
}

function registerUser(Email $email, Role $role, Age $age): void {
    // types carry their own validation — impossible to pass invalid data
}

References

• ↗ https://refactoring.guru/smells/primitive-obsession

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