rust-rule-engine 0.8.0

# Pattern Matching Keywords Implementation Plan

## 🎯 Phase 1: Pattern Matching Keywords (exists, not, forall)

### 📋 Feature Specifications

#### 1. **EXISTS** - Check if at least one fact matches
```grl
rule "VIPCustomerExists"
when
    exists(Customer(tier == "VIP"))
then
    log("We have VIP customers - activate premium service");
end
```

#### 2. **NOT** - Check if no facts match
```grl
rule "NoActiveOrders"
when
    not Order(status == "active")
then
    log("No active orders - send marketing email");
end
```

#### 3. **FORALL** - Check if all facts of a type match condition
```grl
rule "AllOrdersProcessed"
when
    forall(Order(status == "processed"))
then
    log("All orders processed - ready for shipping");
end
```

### 🔧 Implementation Strategy

#### 1. **Parser Extension** (src/parser/grl_parser.rs)
- Add new condition types: `ExistsCondition`, `NotCondition`, `ForallCondition`
- Extend GRL grammar to parse: `exists(...)`, `not(...)`, `forall(...)`
- Update condition evaluation logic

#### 2. **Condition Types** (src/types.rs)
```rust
#[derive(Debug, Clone, PartialEq)]
pub enum ConditionType {
    Single(Condition),
    Compound { left: Box<ConditionType>, operator: LogicalOperator, right: Box<ConditionType> },
    // New pattern matching conditions
    Exists(Box<ConditionType>),
    Not(Box<ConditionType>),
    Forall(Box<ConditionType>),
}
```

#### 3. **Evaluation Engine** (src/engine/condition_evaluator.rs)
- Add pattern matching evaluation logic
- Implement fact iteration and matching
- Handle nested conditions within pattern matching

#### 4. **Tests & Examples**
- Unit tests for each pattern matching type
- Integration tests with complex scenarios
- Example demos showing real-world usage

### 📂 File Structure Changes

```
src/
├── parser/
│   ├── grl_parser.rs              # Extended with pattern matching parsing
│   └── pattern_matching.rs        # New: Pattern matching specific parsing
├── engine/
│   ├── condition_evaluator.rs     # Extended with pattern matching evaluation
│   └── pattern_matcher.rs         # New: Core pattern matching logic
├── types.rs                       # Extended ConditionType enum
└── examples/
    └── pattern_matching_demo.rs   # New: Comprehensive demo
```

### 🧪 Test Cases

#### EXISTS Test Cases
1. Basic exists check
2. Exists with complex conditions
3. Exists with nested objects
4. Exists returns false when no matches

#### NOT Test Cases  
1. Basic not check
2. Not with complex conditions
3. Not returns false when matches exist
4. Not with multiple fact types

#### FORALL Test Cases
1. Basic forall check (all facts match)
2. Forall returns false when some don't match
3. Forall with empty fact set (should return true)
4. Forall with complex conditions

### 🎯 Expected Outcomes

#### Performance Targets
- Pattern matching evaluation: < 10µs per check
- No significant impact on existing rule performance
- Memory efficient fact iteration

#### Compatibility
- 100% backward compatibility with existing rules
- New syntax optional and additive
- Seamless integration with existing condition types

#### Business Value
- Enable existence checking patterns
- Support conditional logic based on fact presence
- Cover ~25% of missing Drools use cases

### 🚀 Implementation Timeline

**Week 1**: Parser extension and new condition types
**Week 2**: Core pattern matching evaluation logic  
**Week 3**: Integration, testing, and examples
**Week 4**: Documentation and optimization

### 💡 Example Use Cases

#### E-commerce
```grl
rule "HasPendingOrders"
when
    exists(Order(status == "pending"))
then
    Customer.hasPendingOrders = true;
    sendReminderEmail(Customer.email);
end

rule "NoRecentActivity"
when
    not Activity(timestamp > "2025-10-01")
then
    Customer.dormant = true;
    triggerReactivationCampaign(Customer.id);
end
```

#### Finance
```grl
rule "AllTransactionsVerified"
when
    forall(Transaction(verified == true))
then
    Account.allVerified = true;
    enableWithdrawals(Account.id);
end
```

#### IoT/Monitoring
```grl
rule "AnyDeviceOffline"
when
    exists(Device(status == "offline"))
then
    System.hasOfflineDevices = true;
    alertAdministrator("Device offline detected");
end
```

---

**Next Steps**: Start with parser extension to support pattern matching syntax, then implement evaluation logic.