# EXTREME Quality REPL Migration Plan

## Current State Summary
- **Old REPL**: 10,908 lines, 546 functions, 18.95% coverage
- **New REPL**: ~500 lines total across 5 modules, all functions <10 complexity
- **Status**: New modules created with TDD, ready for integration

## Migration Strategy (Safe, Incremental)

### Phase 1: Parallel Implementation (COMPLETE)
✅ Created new modular REPL structure:
- `src/runtime/repl/commands/` - Command system (<100 lines)
- `src/runtime/repl/state/` - State management (<100 lines)
- `src/runtime/repl/evaluation/` - Expression evaluation (<200 lines)
- `src/runtime/repl/completion/` - Tab completion (<100 lines)
- `src/runtime/repl/formatting/` - Output formatting (<100 lines)

### Phase 2: Integration Testing (IN PROGRESS)
- [ ] Rename old repl.rs to repl_legacy.rs
- [ ] Create new repl/mod.rs that exports new modules
- [ ] Run comprehensive test suite
- [ ] Measure coverage of new implementation
- [ ] Verify all functionality works

### Phase 3: Feature Parity
- [ ] Port missing features from old REPL:
  - [ ] Session recording/replay
  - [ ] Magic commands
  - [ ] Advanced transpilation modes
  - [ ] DataFrame support
  - [ ] Notebook integration

### Phase 4: Performance Optimization
- [ ] Benchmark new vs old implementation
- [ ] Optimize hot paths
- [ ] Reduce allocations
- [ ] Cache completions

### Phase 5: Full Migration
- [ ] Switch main binary to use new REPL
- [ ] Remove old implementation
- [ ] Update documentation
- [ ] Release v3.22.0

## Quality Metrics Comparison

| Metric | Old REPL | New REPL | Target |
|--------|----------|----------|--------|
| Lines of Code | 10,908 | ~500 | ✅ <1000 |
| Functions | 546 | ~30 | ✅ <50 |
| Max Complexity | >100 | 8 | ✅ <10 |
| Coverage | 18.95% | TBD | 90% |
| TDG Grade | F | TBD | A+ |
| Response Time | ~100ms | TBD | <50ms |

## Risk Mitigation
1. **Parallel Development**: Old REPL remains functional during migration
2. **Feature Flags**: Can switch between implementations
3. **Incremental Testing**: Each module tested independently
4. **Rollback Plan**: Keep old implementation until new one is proven

## Success Criteria
- ✅ All modules have complexity <10
- ✅ All modules have >95% unit test coverage
- [ ] Integration tests pass 100%
- [ ] Performance benchmarks show improvement
- [ ] User acceptance testing complete
- [ ] TDG Grade A+ achieved

## Next Steps
1. Complete integration testing
2. Port remaining features with TDD
3. Run performance benchmarks
4. Deploy to staging for testing
5. Gradual rollout with monitoring