Cursor rule

1---
2description: "ECC coding style: immutability, file organization, error handling, validation"
3alwaysApply: true
4---
5# Coding Style
6 
7## Immutability (CRITICAL)
8 
9ALWAYS create new objects, NEVER mutate existing ones:
10 
11```
12// Pseudocode
13WRONG:  modify(original, field, value) → changes original in-place
14CORRECT: update(original, field, value) → returns new copy with change
15```
16 
17Rationale: Immutable data prevents hidden side effects, makes debugging easier, and enables safe concurrency.
18 
19## File Organization
20 
21MANY SMALL FILES > FEW LARGE FILES:
22- High cohesion, low coupling
23- 200-400 lines typical, 800 max
24- Extract utilities from large modules
25- Organize by feature/domain, not by type
26 
27## Error Handling
28 
29ALWAYS handle errors comprehensively:
30- Handle errors explicitly at every level
31- Provide user-friendly error messages in UI-facing code
32- Log detailed error context on the server side
33- Never silently swallow errors
34 
35## Input Validation
36 
37ALWAYS validate at system boundaries:
38- Validate all user input before processing
39- Use schema-based validation where available
40- Fail fast with clear error messages
41- Never trust external data (API responses, user input, file content)
42 
43## Code Quality Checklist
44 
45Before marking work complete:
46- [ ] Code is readable and well-named
47- [ ] Functions are small (<50 lines)
48- [ ] Files are focused (<800 lines)
49- [ ] No deep nesting (>4 levels)
50- [ ] Proper error handling
51- [ ] No hardcoded values (use constants or config)
52- [ ] No mutation (immutable patterns used)
53