# REQUIRE Keyword Enhancement - Tolerance Block Implementation

## Summary
Successfully implemented conditional tolerance logic for the REQUIRE keyword, allowing developers to define fallback conditions that can override strict requirements based on business logic (VIP status, loyalty points, admin privileges, etc.).

## What Was Implemented

### 1. Tolerance Block Syntax
```zexus
require condition {
    // Tolerance logic executed when condition fails
    if (user.isVIP) {
        print "VIP user - waiving requirement";
        return true;  // Allow despite failing main condition
    } else {
        return false;  // Reject
    }
}
```

### 2. Files Modified

#### parser.py (Line 476)
- **Change**: Commented out old REQUIRE handler to let ContextStackParser handle enhanced syntax
- **Reason**: Old handler only supported `require(condition, message)` syntax

#### strategy_structural.py (Line 437-443)
- **Change**: Added REQUIRE to special brace block handling
- **Reason**: Tolerance blocks `{...}` are part of the statement, not separate data literals

#### strategy_context.py
- **Lines 1900-1920**: Enhanced REQUIRE token collection to track brace nesting
- **Lines 4070-4130**: Enhanced _parse_require_statement to detect and parse tolerance blocks
- **Key Fix**: Search forward for FIRST LBRACE (not backward for last one)
- **Key Fix**: Create BlockStatement correctly without passing statements to constructor

#### statements.py (Lines 2690-2730)
- **Change**: Enhanced eval_require_statement to execute tolerance blocks
- **Logic**: When condition fails, execute tolerance block and unwrap ReturnValue
- **Behavior**: Return NULL if tolerance approves, otherwise continue to error

## Examples That Work

### VIP Fee Waiver
```zexus
let balance = 0.08;  // Below 0.1 minimum
require balance >= 0.1 {
    if (user.isVIP && user.totalSpent >= 100) {
        print "VIP user - waiving fee";
        return true;  // Approved!
    }
    return false;  // Regular users blocked
}
```

### Loyalty Discount
```zexus
require amount >= 100 {
    if (loyaltyPoints >= 500) {
        print "Loyalty discount - minimum waived";
        return true;
    } elif (subscribed) {
        print "Subscriber - minimum waived";
        return true;
    }
    return false;
}
```

### Emergency Override
```zexus
require maintenance == false {
    if (emergency && user == "admin") {
        print "Admin emergency override";
        return true;
    }
    return false;
}
```

## Technical Details

### Parser Flow
1. **Structural Analyzer** (strategy_structural.py):
   - Identifies REQUIRE statement
   - Collects ALL tokens including tolerance block
   - Treats `{...}` as part of statement (not separate)

2. **Context Parser** (strategy_context.py):
   - Receives full token list
   - Detects FIRST LBRACE after condition
   - Extracts and parses tolerance block as BlockStatement
   - Creates RequireStatement with tolerance_block property

3. **Evaluator** (statements.py):
   - Evaluates condition
   - If FALSE, executes tolerance_block
   - Unwraps ReturnValue from block execution
   - If truthy, allows requirement (returns NULL)
   - If falsy, requirement still fails

### Key Insights
- **Parser.py was the blocker**: Old handler caught REQUIRE before strategy_context
- **Forward search crucial**: Finding FIRST LBRACE, not LAST
- **BlockStatement constructor**: Doesn't accept statements parameter, must assign `.statements` after creation
- **ReturnValue unwrapping**: Tolerance block returns ReturnValue object that must be unwrapped

## Testing
- Created test_require_enhanced.zx with 15 comprehensive tests
- All major scenarios working: VIP bypass, loyalty discounts, emergency overrides, tier-based minimums
- Tolerance blocks properly execute and return control flow

## Status
✅ **COMPLETE** - Tolerance blocks fully functional
✅ Parser correctly identifies and extracts tolerance blocks  
✅ Evaluator properly executes and evaluates tolerance logic
✅ All test scenarios passing
