attac-t Skill: Composition
"Favor composition over inheritance."
The Decision
Use Trait + Interface when:
- Behavior crosses inheritance hierarchies
- Multiple behaviors need mixing
- No clear IS-A relationship
- You want horizontal reuse
Use Abstract Class when:
- Clear parent-child relationship exists
- Significant shared implementation
- Template method pattern needed
- You want vertical inheritance
The Heuristic
Ask: "Does this class have this behavior, or is it this thing?"
- Has → Trait
- Is → Abstract Class
The Quick Test
| Ask Yourself | Answer | Use |
|---|---|---|
| Does every subclass need this? | Yes | Abstract |
| Can multiple unrelated classes need this? | Yes | Trait |
| Is there a clear IS-A relationship? | Yes | Abstract |
| Is this a capability/feature? | Yes | Trait |
Real-World Examples
For concrete examples from Laravel, Spatie, and production code, see examples.md.