Fixing Primitive Obsession with Value Objects

Also Known As

TL;DR

Explanation

Primitive obsession uses raw types where domain concepts belong: string $email, float $price, string $status. Problems: no validation enforced, no type safety distinguishing 'email string' from 'name string', business logic scattered across multiple callers. The fix: create small Value Object classes — Email, Money, OrderStatus — that validate and encapsulate the concept. Constructors throw on invalid input, so a valid VO instance is always well-formed. Methods can be added: $money->add($other), $email->domain(). PHP 8.1 readonly classes make VOs concise. PHPStan recognises distinct types, catching swapped-argument bugs statically. Start with the most-duplicated validation logic and extract outward.

Common Misconception

Why It Matters

Common Mistakes

• Value objects with setters — they should be immutable; create a new instance for changes.
• Not overriding __toString() for value objects used in string contexts — causes type errors.
• Value object equality checked with == instead of a dedicated equals() method or comparing properties.
• Wrapping every primitive — only wrap primitives that carry domain rules or validation; plain int IDs often do not need wrapping.

Code Examples

function charge(string $email, float $amount, string $currency): void {
    // Which string is which? float can be negative?
}

function charge(Email $email, Money $amount): void {
    // Email and Money enforce their own invariants
}

References

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

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