Software Engineering Principles

Core Design Principles

SOLID

• Single Responsibility: Each module/class does one thing
• Open/Closed: Extend via new code, not modifying existing
• Liskov Substitution: Subtypes must be substitutable for base types
• Interface Segregation: Many specific interfaces > one general interface
• Dependency Inversion: Depend on abstractions, not concretions

Pragmatic Principles

• YAGNI: Don't build what isn't needed yet
• DRY: No duplicated logic (code or conceptual)
• KISS: Simplest solution that works
• Composition over Inheritance: Prefer has-a over is-a
• Fail Fast, Fail Loud: Errors at the source, not downstream

Code Quality Checklist

• Functions do one thing and are named for what they do
• No function longer than ~30 lines
• No more than 3 parameters per function
• Error handling is explicit (no swallowed errors)
• Dependencies are injected, not hard-coded
• Public interfaces are minimal and well-documented
• Tests test behavior, not implementation

When Principles Conflict

Principles sometimes conflict. Resolution order:

1. Correctness > everything else
2. Simplicity > cleverness
3. Readability > conciseness
4. Explicit > implicit
5. Tested > untested

Design Smell Checklist

Smell	Symptom	Fix
God object	One class does everything	Split by responsibility
Feature envy	Method uses another class's data more than its own	Move method
Shotgun surgery	One change requires touching many files	Extract shared module
Primitive obsession	Using strings/ints where a type would be clearer	Create domain types
Long parameter list	Function takes 5+ params	Create parameter object