# WASM Compilation Limitations Tracker
**Last Updated**: 2025-10-08
**Version**: v3.70.0
**Status**: ✅ MEMORY MODEL COMPLETE - Full support for tuples, structs, and arrays!
## Purpose
This document tracks known WASM compilation limitations to ensure TRANSPARENCY and systematic resolution. Following Toyota Way principles: **STOP THE LINE** when limitations are discovered, document them, then fix them systematically.
## Critical Principle
**NEVER HIDE BUGS OR LIMITATIONS**. Every limitation here represents work to be done. Each limitation blocks real user workflows and must be fixed with EXTREME TDD.
---
## Current Limitations (Active Issues)
### 1. Tuple Destructuring Patterns [WASM-002]
**Status**: ✅ 80% COMPLETE - Real memory loading works! Match patterns not supported
**Priority**: MEDIUM (core functionality working)
**Completed**: v3.70.0 (Phases 1-3 of WASM-005)
**Blocking Tests**:
- `test_langcomp_013_02_basic_destructuring` ✅ WORKS WITH REAL VALUES
- `test_langcomp_013_02_underscore_destructuring` ✅ WORKS WITH REAL VALUES
- `test_langcomp_013_04_destructuring_nested` ✅ WORKS WITH REAL VALUES
- `test_langcomp_013_02_tuple_destructuring_example_file` ⚠️ (blocked by match patterns only)
**Current Behavior**:
```rust
// ✅ WORKS: Basic let destructuring WITH REAL MEMORY LOADS
let (x, y) = (3, 4)
println(x) // Prints: 3 (loaded from memory address 0)
println(y) // Prints: 4 (loaded from memory address 4)
// ✅ WORKS: Nested destructuring WITH REAL MEMORY
let ((a, b), c) = ((1, 2), 3)
println(a) // Prints: 1 (loaded from inner tuple memory)
println(b) // Prints: 2
println(c) // Prints: 3 (loaded from outer tuple memory)
// ✅ WORKS: Underscore patterns
let (x, _, z) = (1, 2, 3)
println(x) // Prints: 1 (real value)
println(z) // Prints: 3 (real value)
// ❌ ONLY LIMITATION: Pattern variables in match arms
match point {
(x, y) => println(x) // Not supported (requires scoped locals)
}
```
**Root Cause**:
- ✅ `lower_let_pattern()` loads actual values from tuple memory (v3.70.0)
- ✅ `store_pattern_values()` uses i32.load to extract tuple elements
- ✅ Bump allocator allocates real memory for tuples
- ✅ Nested tuples work correctly
- ❌ Match arm pattern variables require scoped locals (WASM architectural limitation)
**Implementation Complete (v3.70.0)**:
- Handle Pattern::Tuple in let bindings ✅
- Allocate memory for tuples using bump allocator ✅
- Load actual values from tuple memory (NOT placeholders!) ✅
- Support nested destructuring ✅
- Support underscore patterns ✅
- Match arm patterns: Intentionally not supported (requires scoped locals)
**Memory Model (WASM-005 Phases 1-3)**:
```
Example: let (x, y) = (3, 4)
1. Allocate tuple at address 0 (8 bytes)
Memory[0-3]: 3
Memory[4-7]: 4
2. Destructure with i32.load
x = i32.load(address + 0) = 3
y = i32.load(address + 4) = 4
```
**Remaining Work**:
- Match pattern bindings: Requires scoped locals architecture (future work, low priority)
**Test Files**:
- ✅ test_destructure_real.ruchy - PASSES
- ✅ test_nested_destructure.ruchy - PASSES
- ⚠️ examples/lang_comp/13-tuples/02_tuple_destructuring.ruchy (match patterns only)
---
### 2. Struct Field Mutation [WASM-003]
**Status**: ✅ COMPLETE - Real struct field mutation working! (v3.70.0 Phase 4)
**Priority**: COMPLETE
**Completed**: v3.70.0 (Phase 4 - Struct Registry & Field Mutation)
**Current Behavior**:
```rust
// ✅ WORKS: Struct definition and instantiation
struct Point { x: i32, y: i32 }
let mut p = Point { x: 3, y: 4 }
println(p.x) // Prints: 3 (loaded from memory offset 0)
println(p.y) // Prints: 4 (loaded from memory offset 4)
// ✅ WORKS: Field mutation stores to memory
p.x = 10
println(p.x) // Prints: 10 (real mutation!)
// ✅ WORKS: Field access after mutation
println(p.y) // Prints: 4 (unchanged)
```
**Implementation Complete (v3.70.0 Phase 4)**:
- ✅ Struct registry: Maps struct_name → field_names for offset calculation
- ✅ collect_struct_definitions(): Traverses AST to collect struct definitions
- ✅ lower_struct_literal(): Allocates memory with bump allocator, returns address
- ✅ lower_field_access(): Looks up field offset from registry, uses i32.load
- ✅ lower_assign() for FieldAccess: Uses i32.store at calculated offset
- ✅ Five Whys root cause analysis: Fixed missing struct registry architecture
**Memory Model**:
```
struct Point { x: i32, y: i32 }
let p = Point { x: 3, y: 4 }
1. Allocate 8 bytes at address 0 (2 fields × 4 bytes)
Memory[0-3]: 3 (field x at offset 0)
Memory[4-7]: 4 (field y at offset 4)
2. Field access: p.x
i32.load(address + 0) = 3
3. Field mutation: p.x = 10
i32.store(address + 0, 10)
```
**Limitations**:
- ⚠️ Nested field mutation `obj.field.subfield = val` requires chained address calculation (future work)
- ⚠️ Field name collisions across structs use heuristic (first match wins)
**Test Files**:
- test_struct_mutation.ruchy ✅ PASSING (compiles to valid 146-byte WASM module)
---
### 3. Complex Assignment Targets [WASM-004]
**Status**: ✅ COMPLETE - All assignment targets working! (v3.70.0 Phases 4-5)
**Priority**: COMPLETE
**Completed**: v3.70.0 (Phase 4: Fields, Phase 5: Arrays/Tuples)
**Current Behavior**:
```rust
// ✅ WORKS: Array element assignment
let mut arr = [10, 20, 30]
arr[0] = 100
println(arr[0]) // Prints: 100 (real mutation!)
// ✅ WORKS: Field assignment
struct Point { x: i32, y: i32 }
let mut p = Point { x: 3, y: 4 }
p.x = 10
println(p.x) // Prints: 10 (real mutation!)
// ✅ WORKS: Tuple element assignment (using field syntax)
let mut tup = (1, 2, 3)
tup.0 = 100
println(tup.0) // Prints: 100 (real mutation!)
```
**Implementation Complete (v3.70.0)**:
- ✅ Field assignment (Phase 4): `obj.field = val` with i32.store
- ✅ Array element assignment (Phase 5): `arr[i] = val` with dynamic offset
- ✅ Tuple element assignment (Phase 5): `tup.0 = val` works via field access
- ✅ Dynamic indexing: Runtime offset computation with i32.mul
- ✅ Memory stores: All mutations persist in memory
**Limitations**:
- ⚠️ Nested complex assignments `arr[i].field = val` require chained address calculation (future work)
**Test Files**:
- test_array_mutation.ruchy ✅ PASSING (204-byte WASM module)
- test_struct_mutation.ruchy ✅ PASSING (146-byte WASM module)
---
### 4. Full Memory Model [WASM-005]
**Status**: ✅ PHASES 1-5 COMPLETE - Full memory model working! (v3.70.0)
**Priority**: COMPLETE
**Completed**: Bump allocator, tuples, structs, arrays with full mutation support
**Current Behavior**:
```rust
// ✅ WORKING: Tuples use real memory
let pair = (3, 4) // Allocates 8 bytes, returns address 0
let first = pair.0 // i32.load(0) = 3 (REAL value!)
let (x, y) = pair // Loads x=3, y=4 from memory
// ✅ WORKING: Structs use real memory
let s = Point { x: 1, y: 2 } // Allocates 8 bytes, returns address
s.x = 10 // i32.store with field offset
// ✅ WORKING: Arrays use real memory
let arr = [1, 2, 3] // Allocates 12 bytes, returns address
arr[0] = 100 // i32.store with dynamic offset
```
**Completed Implementation (v3.70.0)**:
- ✅ **Phase 1**: Memory foundation (64KB heap, global $heap_ptr)
- ✅ **Phase 2**: Tuple storage (allocate, store, return address)
- ✅ **Phase 3**: Tuple destructuring (i32.load elements into locals)
- ✅ **Phase 4**: Struct field mutation (struct registry, field offset calculation)
- ✅ **Phase 5**: Array element access (dynamic indexing with i32.mul)
**Design Decisions (Implemented)**:
- ✅ Bump allocator: O(1) allocation, no GC/free (acceptable for MVP)
- ✅ Fixed 64KB heap: Sufficient for testing
- ✅ Sequential layout: No padding, no type tags
- ✅ i32 only: All values are 4 bytes
- ✅ Dynamic offsets: Runtime index * 4 for arrays
**Future Work (Low Priority)**:
- Garbage collection (not needed for short-running programs)
- Growing heap (64KB sufficient for current use cases)
- Nested complex assignments: `arr[i].field = val`
**Memory Model Architecture**:
```wat
;; Global heap pointer (mutable)
(global $heap_ptr (mut i32) (i32.const 0))
;; Inline bump allocator in lower_tuple()
global.get $heap_ptr ;; Get current address
local.set $temp ;; Save it
global.get $heap_ptr
i32.const 8 ;; Size needed
i32.add
global.set $heap_ptr ;; Update heap pointer
;; Store elements
local.get $temp
i32.const 3
i32.store offset=0 ;; Store first element
local.get $temp
i32.const 4
i32.store offset=4 ;; Store second element
local.get $temp ;; Return address
```
---
## Completed Features
### ✅ Memory Model Foundation [WASM-005 Phase 1] (v3.70.0)
- **Implemented**: Commit 9a4a67ae
- **Status**: FULLY WORKING - 64KB heap with global heap pointer
- **Features**:
- Memory section: 1 page (64KB), max=1
- Global section: `$heap_ptr` (mutable i32, init=0)
- Design document: docs/execution/WASM_MEMORY_MODEL.md
- **Test**: Memory and global sections present in all tuple code
### ✅ Tuple Memory Storage [WASM-005 Phase 2] (v3.70.0)
- **Implemented**: Commit f7fdb1de
- **Status**: FULLY WORKING - Real memory allocation and storage
- **Features**:
- Inline bump allocator in `lower_tuple()`
- Allocates memory: GlobalGet($heap_ptr) → save → update → store
- Returns memory address instead of placeholder
- Field access loads from memory with i32.load
- **Test**: `let pair = (3, 4); println(pair.0)` prints 3 (real value!)
### ✅ Tuple Destructuring [WASM-005 Phase 3] (v3.70.0)
- **Implemented**: Commit 30089fc6
- **Status**: FULLY WORKING - Loads real values from memory
- **Features**:
- `store_pattern_values()` loads from tuple memory
- Uses i32.load at address + offset for each element
- Stores loaded values into pattern variable locals
- Nested destructuring works correctly
- **Test**: `let (x, y) = (3, 4); println(x)` prints 3 (real value!)
### ✅ Struct Field Mutation [WASM-005 Phase 4] (v3.70.0)
- **Implemented**: Commit 4a42b76a
- **Status**: FULLY WORKING - Real struct field mutation with memory stores
- **Features**:
- Struct registry: Maps struct_name → field_names for offset calculation
- `collect_struct_definitions()`: Traverses AST to collect struct definitions
- `lower_struct_literal()`: Allocates memory with bump allocator
- `lower_field_access()`: Looks up field offset from registry, uses i32.load
- `lower_assign()` for FieldAccess: Uses i32.store at calculated offset
- Five Whys root cause analysis: Fixed missing struct registry architecture
- **Test**: `p.x = 10` stores to memory, `println(p.x)` prints 10 (real mutation!)
### ✅ Array Element Access [WASM-005 Phase 5] (v3.70.0)
- **Implemented**: Commit 27bb8474
- **Status**: FULLY WORKING - Dynamic array indexing with memory loads/stores
- **Features**:
- `lower_list()`: Allocates memory for arrays with bump allocator
- `lower_index_access()`: Dynamic offset computation (index * 4) with i32.mul
- `lower_assign()` for IndexAccess: Dynamic i32.store at runtime-computed address
- Runtime indexing: Supports variable indices, not just constants
- Works for both arrays and tuples with numeric indices
- **Test**: `arr[0] = 100; println(arr[0])` prints 100 (real dynamic mutation!)
### ✅ Basic Tuple Creation [WASM-001] (v3.69.0 → v3.70.0)
- **Implemented**: Commit d43197f2 → Upgraded f7fdb1de
- **Status**: FULLY WORKING WITH REAL MEMORY (v3.70.0)
- **Test**: `test_langcomp_013_01_basic_tuples_example_file` ✅ PASSES
### ✅ Tuple Field Access [WASM-001] (v3.69.0 → v3.70.0)
- **Implemented**: Commit d43197f2 → Upgraded f7fdb1de
- **Status**: FULLY WORKING WITH REAL MEMORY (v3.70.0)
- **Test**: Loads actual values from memory with i32.load
### ✅ Simple Assignment [WASM-001] (v3.69.0)
- **Implemented**: Commit d43197f2
- **Status**: WORKING
- **Test**: `coords = (5, 10)` compiles successfully
### ✅ Unit Literal [WASM-001] (v3.69.0)
- **Implemented**: Commit d43197f2
- **Status**: WORKING
- **Test**: `let unit = ()` compiles successfully
### ✅ Character Literals [WASM-001] (v3.69.0)
- **Implemented**: Commit d43197f2
- **Status**: WORKING (UTF-32 code points)
- **Test**: `'a'` compiles to I32Const(97)
---
## Implementation Strategy (EXTREME TDD)
### Phase 1: Tuple Destructuring [WASM-002]
1. **RED**: Write failing test for `let (x, y) = (1, 2)`
2. **GREEN**: Implement Pattern::Tuple handling in lower_let()
3. **REFACTOR**: Ensure complexity <10, add comprehensive tests
4. **VALIDATE**: Run mutation tests, property tests
5. **COMMIT**: Only after all tests pass
### Phase 2: Struct Field Mutation [WASM-003]
1. **RED**: Write failing test for `p.x = 5`
2. **GREEN**: Implement FieldAccess in lower_assign()
3. **REFACTOR**: Handle nested fields
4. **VALIDATE**: Mutation testing
5. **COMMIT**: Only after all tests pass
### Phase 3: Memory Model [WASM-005]
1. **DESIGN**: Document memory layout strategy
2. **RED**: Write tests for actual data storage/retrieval
3. **GREEN**: Implement linear memory allocation
4. **REFACTOR**: Optimize for common patterns
5. **VALIDATE**: Property tests for memory safety
6. **COMMIT**: Only after comprehensive validation
---
## Test Coverage Requirements
Each limitation fix MUST include:
- ✅ Unit tests for specific feature
- ✅ Integration tests with LANG-COMP examples
- ✅ Property tests (10K+ random inputs)
- ✅ Mutation tests (75%+ coverage)
- ✅ Negative tests (error handling)
- ✅ Edge cases (empty tuples, nested patterns, etc.)
---
## Toyota Way Principles
1. **Jidoka (Stop the Line)**:
- When WASM compilation fails, STOP and fix immediately
- Don't defer to "future work" - fix it NOW
- Document limitation → Implement fix → Validate → Ship
2. **Genchi Genbutsu (Go and See)**:
- Run actual LANG-COMP tests to see real failures
- Use failing tests to drive implementation
- Never assume - always verify with tests
3. **Kaizen (Continuous Improvement)**:
- MVP → Basic → Full implementation
- Each commit adds working functionality
- No half-finished features in main branch
4. **Respect for People**:
- Clear documentation of what works and what doesn't
- No surprises for users
- Honest about limitations
---
## Acceptance Criteria
A limitation is considered RESOLVED when:
1. ✅ All blocking LANG-COMP tests pass
2. ✅ Property tests validate correctness (10K+ cases)
3. ✅ Mutation tests achieve 75%+ coverage
4. ✅ Documentation updated
5. ✅ Examples work end-to-end
6. ✅ No regressions in existing features
---
## Next Steps
### Completed (v3.69.x)
1. ✅ **[WASM-002]** Tuple Destructuring MVP (Commit 38096bfb)
- Basic let destructuring works: `let (x, y) = (1, 2)`
- Match pattern bindings not supported (documented limitation)
2. ✅ **[WASM-003]** Struct Field Mutation MVP (Commit 5e8a8042)
- Syntax compiles: `p.x = 5`
- Values dropped (no actual mutation until memory model)
- Honest documentation prevents user confusion
3. ✅ **[WASM-004]** Complex Assignment Targets MVP (Current session)
- Array element assignment: `arr[0] = 10` ✅ COMPILES
- Tuple element assignment: `tup.0 = 3` ✅ COMPILES
- Added lower_index_access() for reading array/tuple elements
- All assignment targets now supported (placeholders)
- Complexity maintained <10 for all functions
### Current Priorities
1. **CRITICAL BLOCKER**: Design [WASM-005] Memory Model
- Research WASM linear memory best practices
- Document design decisions:
* Manual memory management vs GC?
* Fixed-size heap vs growing heap?
* Type tagging strategy for dynamic types?
- Implement incrementally with EXTREME TDD
- This unblocks ACTUAL implementation of WASM-002, WASM-003, WASM-004
3. **AFTER MEMORY MODEL**: Convert MVPs to Real Implementations
- Replace I32Const(0) placeholders with actual data
- Implement tuple unpacking from memory
- Implement struct field access from memory
- Implement array element access from memory
- Full validation with property tests + mutation tests
---
**Remember**: Every limitation here represents a defect. **STOP THE LINE** and fix them systematically. No shortcuts. No deferrals. Quality is built in, not bolted on.