# Fix Summary: Function Declaration Support in Zexus

## Issue Resolved
**Error**: `❌ Error: Identifier 'function' not found` when executing `test_phase10_ecosystem.zx`

## Root Cause
The `function` keyword was not recognized by the Zexus language - it wasn't tokenized as a keyword and had no parsing or evaluation rules.

## Solution: 6-Step Implementation

### Step 1: Token Definition
**File**: `src/zexus/zexus_token.py`
- Added: `FUNCTION = "FUNCTION"` token constant

### Step 2: Lexer Keywords
**File**: `src/zexus/lexer.py`
- Added: `"function": FUNCTION` to the keywords dictionary in `lookup_ident()`

### Step 3: AST Node
**File**: `src/zexus/zexus_ast.py`
- Added: `FunctionStatement` class (mirrors `ActionStatement` structure)

### Step 4: Parser Rules
**File**: `src/zexus/parser/parser.py`
- Added: `elif self.cur_token_is(FUNCTION): node = self.parse_function_statement()` in `parse_statement()`
- Added: `parse_function_statement()` method (identical logic to `parse_action_statement()`)

### Step 5: Evaluator Support - Core
**File**: `src/zexus/evaluator/core.py`
- Added: Case to dispatch `FunctionStatement` nodes to `eval_function_statement()`

### Step 6: Evaluator Support - Statements
**File**: `src/zexus/evaluator/statements.py`
- Added: `FunctionStatement` to imports
- Added: `eval_function_statement()` method (identical logic to `eval_action_statement()`)

## Test Results

### Original Failing Test: ✅ FIXED
```
File: src/tests/test_phase10_ecosystem.zx
Size: 5086 bytes, 177 lines

✅ Tokenization: SUCCESS
✅ Parsing: 90 statements parsed (10 FunctionStatements)
✅ Evaluation: SUCCESS - all 10 functions registered

Functions defined:
  ✓ registerPackage
  ✓ installPackage
  ✓ resolveDependencies
  ✓ searchMarketplace
  ✓ getMarketplaceStats
  ✓ profileFunction
  ✓ recordMetric
  ✓ checkVersionCompatibility
  ✓ upgradePackage
  ✓ uninstallPackage
```

### Additional Validation Tests: ✅ ALL PASS
1. ✅ Simple function declarations
2. ✅ Functions with parameters
3. ✅ Multiple function declarations
4. ✅ Functions with complex bodies
5. ✅ Full Phase 10 ecosystem test

## Technical Details

### Design Pattern
- **Reusability**: `FunctionStatement` evaluation uses the same `Action` object as `ActionStatement`
- **Consistency**: Function declarations have identical semantics to action declarations
- **Compatibility**: All action modifiers work with functions (`async`, `secure`, `pure`, `inline`, `native`)
- **Equivalence**: Both `function` and `action` keywords produce identical runtime behavior

### Code Changes Summary
- **Files modified**: 6
- **Lines added**: ~80
- **Lines removed**: 0
- **Breaking changes**: None
- **Backward compatible**: Yes ✅

## Verification

✅ **Lexer verification**: `"function"` correctly tokenized as `FUNCTION`
✅ **Parser verification**: Creates `FunctionStatement` AST nodes
✅ **Evaluator verification**: Functions register in environment
✅ **Integration verification**: Full test suite passes
✅ **Phase 10 test**: Originally failing test now passes

## Impact Assessment

| Aspect | Impact |
|--------|--------|
| **Breaking Changes** | None |
| **Backward Compatibility** | Full (actions still work) |
| **Performance** | No change |
| **New Capabilities** | Standard `function` keyword now supported |
| **Test Coverage** | Comprehensive (unit + integration) |
| **Documentation** | See FUNCTION_DECLARATION_FIX.md |

## Conclusion

The Zexus interpreter now fully supports standard `function` keyword declarations alongside the original `action` keyword. Both syntaxes are functionally equivalent and fully supported throughout the language.

The error `"Identifier 'function' not found"` has been completely resolved.
