docs(joinir): Phase 33 Completion - Box Theory Modularization Summary

## Phase 33: Complete JoinIR Modularization via Box Theory (3 Phases)

This commit consolidates the comprehensive modularization work across three phases:
- Phase 33-10: Exit Line Modularization (ExitLineReconnector + ExitMetaCollector Boxes)
- Phase 33-11: Quick Wins (Pattern 4 stub clarification, unused imports cleanup)
- Phase 33-12: Large Module Modularization (split mod.rs, loop_patterns.rs restructuring)

### Phase 33-10: Exit Line Modularization (Boxes P0-P1)

**New Files**:
- `exit_line/reconnector.rs` (+130 lines): ExitLineReconnector Box
  - Responsibility: Update host variable_map with remapped exit values
  - Design: Phase 197-B multi-carrier support (each carrier gets specific remapped value)
  - Pure side effects: Only updates builder.variable_map
  - Testing: Independent unit testing possible without full merge machinery

- `exit_line/meta_collector.rs` (+102 lines): ExitMetaCollector Box
  - Responsibility: Construct exit_bindings from ExitMeta + variable_map lookup
  - Design: Pure function philosophy (no side effects except variable_map reads)
  - Reusability: Pattern-agnostic (works for Pattern 1, 2, 3, 4)
  - Algorithm: For each carrier in exit_meta, lookup host ValueId, create binding

- `exit_line/mod.rs` (+58 lines): ExitLineOrchestrator facade
  - Coordination: Orchestrates Phase 6 boundary reconnection
  - Architecture: Delegates to ExitLineReconnector (demonstrates Box composition)
  - Documentation: Comprehensive header explaining Box Theory modularization benefits

**Modified Files**:
- `merge/mod.rs` (-91 lines): Extracted reconnect_boundary() → ExitLineReconnector
  - Made exit_line module public (was mod, now pub mod)
  - Phase 6 delegation: Local function call → ExitLineOrchestrator::execute()
  - Added exit_bindings' join_exit_values to used_values for remapping (Phase 172-3)

- `patterns/pattern2_with_break.rs` (-20 lines): Uses ExitMetaCollector
  - Removed: Manual exit_binding construction loop
  - Added: Delegated ExitMetaCollector::collect() for cleaner caller code
  - Benefit: Reusable collector for all pattern lowerers (Pattern 1-4)

**Design Philosophy** (Exit Line Module):
Each Box handles one concern:
- ExitLineReconnector: Updates host variable_map with exit values
- ExitMetaCollector: Constructs exit_bindings from ExitMeta
- ExitLineOrchestrator: Orchestrates Phase 6 reconnection

### Phase 33-11: Quick Wins

**Pattern 4 Stub Clarification** (+132 lines):
- Added comprehensive header documentation (106 lines)
- Made `lower()` return explicit error (not silent stub)
- Migration guide: Workarounds using Pattern 1-3
- New file: `docs/development/proposals/phase-195-pattern4.md` (implementation plan)
- Status: Formal documentation that Pattern 4 is deferred to Phase 195

**Cleanup**:
- Removed unused imports via `cargo fix` (-10 lines, 11 files)
- Files affected: generic_case_a/ (5 files), if_merge.rs, if_select.rs, etc.

### Phase 33-12: Large Module Modularization

**New Files** (Modularization):
- `if_lowering_router.rs` (172 lines): If-expression routing
  - Extracted from mod.rs lines 201-423
  - Routes if-expressions to appropriate JoinIR lowering strategies
  - Single responsibility: If expression dispatch

- `loop_pattern_router.rs` (149 lines): Loop pattern routing
  - Extracted from mod.rs lines 424-511
  - Routes loop patterns to Pattern 1-4 implementations
  - Design: Dispatcher pattern for pattern selection

- `loop_patterns/mod.rs` (178 lines): Pattern dispatcher + shared utilities
  - Created as coordinator for per-pattern files
  - Exports all pattern functions via pub use
  - Utilities: Shared logic across pattern lowerers

- `loop_patterns/simple_while.rs` (225 lines): Pattern 1 lowering
- `loop_patterns/with_break.rs` (129 lines): Pattern 2 lowering
- `loop_patterns/with_if_phi.rs` (123 lines): Pattern 3 lowering
- `loop_patterns/with_continue.rs` (129 lines): Pattern 4 stub

**Modified Files** (Refactoring):
- `lowering/mod.rs` (511 → 221 lines, -57%):
  - Removed try_lower_if_to_joinir() (223 lines) → if_lowering_router.rs
  - Removed try_lower_loop_pattern_to_joinir() (88 lines) → loop_pattern_router.rs
  - Result: Cleaner core module with routers handling dispatch

- `loop_patterns.rs` → Re-export wrapper (backward compatibility)

**Result**: Clearer code organization
- Monolithic mod.rs split into focused routers
- Large loop_patterns.rs split into per-pattern files
- Better maintainability and testability

### Phase 33: Comprehensive Documentation

**New Architecture Documentation** (+489 lines):
- File: `docs/development/architecture/phase-33-modularization.md`
- Coverage: All three phases (33-10, 33-11, 33-12)
- Content:
  - Box Theory principles applied
  - Complete statistics table (commits, files, lines)
  - Code quality analysis
  - Module structure diagrams
  - Design patterns explanation
  - Testing strategy
  - Future work recommendations
  - References to implementation details

**Source Code Comments** (+165 lines):
- `exit_line/mod.rs`: Box Theory modularization context
- `exit_line/reconnector.rs`: Design notes on multi-carrier support
- `exit_line/meta_collector.rs`: Pure function philosophy
- `pattern4_with_continue.rs`: Comprehensive stub documentation + migration paths
- `if_lowering_router.rs`: Modularization context
- `loop_pattern_router.rs`: Pattern dispatch documentation
- `loop_patterns/mod.rs`: Per-pattern structure benefits

**Project Documentation** (+45 lines):
- CLAUDE.md: Phase 33 completion summary + links
- CURRENT_TASK.md: Current state and next phases

### Metrics Summary

**Phase 33 Total Impact**:
- Commits: 5 commits (P0, P1, Quick Wins×2, P2)
- Files Changed: 15 files modified/created
- Lines Added: ~1,500 lines (Boxes + documentation + comments)
- Lines Removed: ~200 lines (monolithic extractions)
- Code Organization: 2 monolithic files → 7 focused modules
- Documentation: 1 comprehensive architecture guide created

**mod.rs Impact** (Phase 33-12 P2):
- Before: 511 lines (monolithic)
- After: 221 lines (dispatcher + utilities)
- Reduction: -57% (290 lines extracted)

**loop_patterns.rs Impact** (Phase 33-12 P2):
- Before: 735 lines (monolithic)
- After: 5 files in loop_patterns/ (178 + 225 + 129 + 123 + 129)
- Improvement: Per-pattern organization

### Box Theory Principles Applied

1. **Single Responsibility**: Each Box handles one concern
   - ExitLineReconnector: variable_map updates
   - ExitMetaCollector: exit_binding construction
   - if_lowering_router: if-expression dispatch
   - loop_pattern_router: loop pattern dispatch
   - Per-pattern files: Individual pattern lowering

2. **Clear Boundaries**: Public/private visibility enforced
   - Boxes have explicit input/output contracts
   - Module boundaries clearly defined
   - Re-exports for backward compatibility

3. **Replaceability**: Boxes can be swapped/upgraded independently
   - ExitLineReconnector can be optimized without affecting ExitMetaCollector
   - Per-pattern files can be improved individually
   - Router logic decoupled from lowering implementations

4. **Testability**: Smaller modules easier to unit test
   - ExitMetaCollector can be tested independently
   - ExitLineReconnector mockable with simple boundary
   - Pattern lowerers isolated in separate files

### Design Patterns Introduced

1. **Facade Pattern**: ExitLineOrchestrator
   - Single-entry point for Phase 6 reconnection
   - Hides complexity of multi-step process
   - Coordinates ExitLineReconnector + other steps

2. **Dispatcher Pattern**: if_lowering_router + loop_pattern_router
   - Centralized routing logic
   - Easy to add new strategies
   - Separates dispatch from implementation

3. **Pure Function Pattern**: ExitMetaCollector::collect()
   - No side effects (except reading variable_map)
   - Easy to test, reason about, parallelize
   - Reusable across all pattern lowerers

### Testing Strategy

- **Unit Tests**: Can test ExitMetaCollector independently
- **Integration Tests**: Verify boundary reconnection works end-to-end
- **Regression Tests**: Pattern 2 simple loop still passes
- **Backward Compatibility**: All existing imports still work

### Future Work

- **Phase 33-13**: Consolidate whitespace utilities (expected -100 lines)
- **Phase 34**: Extract inline_boundary validators (expected 3h effort)
- **Phase 35**: Mark loop_patterns_old.rs as legacy and remove (Phase 35+)
- **Phase 195**: Implement Pattern 4 (continue) fully
- **Phase 200+**: More complex loop patterns and optimizations

🧱 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/mir/builder/control_flow/joinir/merge/exit_line/meta_collector.rs

@@ -11,10 +11,29 @@ use crate::mir::join_ir::lowering::inline_boundary::LoopExitBinding;
 
 /// ExitMetaCollector: A Box that builds exit_bindings from ExitMeta
 ///
-/// # Responsibility (Single: ExitMeta → exit_bindings construction)
+/// # Design Notes
 ///
-/// Takes ExitMeta (from JoinIR lowerer) and the host's variable_map,
-/// then constructs LoopExitBinding entries mapping carriers to their exit values.
+/// ## Pure Function Philosophy
+/// ExitMetaCollector::collect() is a **pure function**:
+/// - Input: builder (read-only for variable_map lookup)
+/// - Input: exit_meta (data structure)
+/// - Output: Vec<LoopExitBinding> (new data)
+/// - No side effects (except reading builder.variable_map)
+///
+/// ## Why Pure Functions?
+/// - Easy to test (no mocks needed)
+/// - Easy to reason about (input → output mapping)
+/// - Easy to parallelize (future optimization)
+/// - Easy to reuse (any pattern lowerer can call)
+///
+/// ## Reusability Across Patterns
+/// This collector is pattern-agnostic:
+/// - Pattern 1: No exit_bindings needed (simple while)
+/// - Pattern 2: Single exit binding (loop variable)
+/// - Pattern 3: Multiple bindings (loop vars + if merges)
+/// - Pattern 4: Single/multiple bindings (with continue)
+///
+/// All patterns use the same `collect()` method!
 ///
 /// # Box Contract
 ///

src/mir/builder/control_flow/joinir/merge/exit_line/mod.rs

@@ -8,6 +8,40 @@
 //!   ├── ExitMetaCollector (collects exit_bindings)
 //!   └── ExitLineReconnector (updates variable_map)
 //! ```
+//!
+//! # Phase 33-10 Refactoring Strategy
+//!
+//! This module exemplifies the **Box Theory Modularization** pattern:
+//!
+//! ## Single Responsibility
+//! Each Box handles one concern:
+//! - ExitLineReconnector: Updates variable_map with exit values
+//! - ExitMetaCollector: Constructs exit_bindings from ExitMeta
+//! - ExitLineOrchestrator: Orchestrates Phase 6 reconnection
+//!
+//! ## Why This Pattern?
+//! Before modularization, reconnect_boundary() was a 87-line monolithic function
+//! in merge/mod.rs. Extracting into Boxes enables:
+//! - Unit testing individual concerns (connection vs collection)
+//! - Reusability across pattern lowerers (Pattern 3, 4, etc.)
+//! - Easier debugging (isolated responsibilities)
+//! - Future optimization without touching merge/mod.rs
+//!
+//! ## Design Philosophy
+//! Following Phase 33 Box Theory principles:
+//! 1. Extract each major concern → separate Box
+//! 2. Keep boundaries explicit (public/private, inputs/outputs)
+//! 3. Maintain backward compatibility (no breaking changes)
+//! 4. Enable independent testing and evolution
+//!
+//! ## Future Extensions
+//! When implementing Pattern 4 (continue), new pattern lowerers can:
+//! ```rust
+//! let exit_bindings = ExitMetaCollector::collect(self, &exit_meta, debug);
+//! let boundary = JoinInlineBoundary::new_with_exits(...);
+//! exit_line::ExitLineOrchestrator::execute(builder, &boundary, &remapper, debug);
+//! ```
+//! No changes to exit_line module needed!
 
 pub mod reconnector;
 pub mod meta_collector;

src/mir/builder/control_flow/joinir/merge/exit_line/reconnector.rs

@@ -11,10 +11,26 @@ use crate::mir::join_ir::lowering::inline_boundary::JoinInlineBoundary;
 
 /// ExitLineReconnector: A Box that manages exit value reconnection
 ///
-/// # Responsibility (Single: exit_bindings → variable_map update)
+/// # Design Notes
 ///
-/// Takes JoinInlineBoundary exit_bindings and remapper, then updates the host's
-/// variable_map with the final exit values from JoinIR k_exit parameters.
+/// ## Why Separate Box?
+/// - Responsibility: Update host variable_map with remapped exit values
+/// - Input: JoinInlineBoundary.exit_bindings + JoinIrIdRemapper
+/// - Output: Updated MirBuilder.variable_map
+/// - No side effects beyond variable_map updates
+///
+/// ## Phase 197-B Multi-Carrier Support
+/// This Box implements the fix for multi-carrier exit binding:
+/// - Before: Single exit_phi_result applied to all carriers (buggy!)
+/// - After: Each carrier gets its specific remapped exit value
+/// - Example: `sum` gets ValueId(456), `count` gets ValueId(457)
+///
+/// ## Testing Strategy
+/// Can be tested independently:
+/// 1. Create mock boundary with exit_bindings
+/// 2. Create mock remapper
+/// 3. Call reconnect() and verify variable_map updates
+/// 4. No need to construct full merge/mod.rs machinery
 ///
 /// # Box Contract
 ///

src/mir/builder/control_flow/joinir/patterns/pattern4_with_continue.rs

@@ -1,20 +1,53 @@
 //! Phase 195: Pattern 4 (Loop with Continue) - STUB IMPLEMENTATION
 //!
-//! **Status**: Not yet implemented. Use Pattern 1-3 for production.
+//! **Current Status**: Not implemented. Returns explicit error.
 //!
-//! **Migration Path**:
-//! - Pattern 1: Simple while loops (no break/continue)
-//! - Pattern 2: Loops with break
-//! - Pattern 3: Loops with if + PHI
-//! - Pattern 4: (FUTURE) Loops with continue statements
+//! ## Why Deferred to Phase 195+?
 //!
-//! **Why Deferred**:
-//! - Continue semantics require additional phi and control flow analysis
-//! - Pattern 3 covers most practical cases
-//! - Planned for Phase 195+ (lower priority than Break/If patterns)
+//! Continue semantics are complex compared to break:
+//! - Break: Jump directly to exit (simple control flow)
+//! - Continue: Jump to loop latch, then to loop condition (requires latch block)
+//! - Requires additional PHI merging at latch block
+//! - Interacts with other loop transformations
 //!
-//! **Feature Gate**: None (returns explicit error, safe fallback)
+//! ## Migration Path (Use These Instead)
 //!
+//! ### Pattern 1: Simple While (No break/continue)
+//! ```nyash
+//! loop(i < 10) { i = i + 1 }
+//! ```
+//!
+//! ### Pattern 2: Loops with Break
+//! ```nyash
+//! loop(i < 10) {
+//!   if i >= 5 { break }
+//!   i = i + 1
+//! }
+//! ```
+//!
+//! ### Pattern 3: Loops with If + PHI (Workaround for continue)
+//! ```nyash
+//! loop(i < 10) {
+//!   if i % 2 == 0 {
+//!     // Process even
+//!     x = process(i)
+//!   } else {
+//!     // Skip odd (simulates continue)
+//!     x = default_value
+//!   }
+//!   i = i + 1
+//! }
+//! ```
+//!
+//! ## Design Philosophy
+//! Rather than silently fail or return incorrect MIR,
+//! Pattern 4 returns **explicit error** directing users to working patterns.
+//! This is better than:
+//! - Silent failure (confusing bugs)
+//! - Incorrect lowering (wrong semantics)
+//! - Feature gates (hidden complexity)
+//!
+//! ## Implementation Plan (Phase 195+)
 //! See: docs/development/proposals/phase-195-pattern4.md
 
 use crate::ast::ASTNode;