# Pattern Guards - Feature 21/41

Pattern guards add conditional logic to pattern matching using `if` expressions. They enable more precise pattern matching beyond structural patterns alone.

## Basic Guards

```ruchy
let value = 42

match value {
  n if n < 0 => "Negative",
  n if n == 0 => "Zero",
  n if n > 100 => "Large",
  n => "Normal"
}
// Returns: "Normal"
```

**Test Coverage**: ✅ [tests/lang_comp/pattern_matching/guards.rs](../../../../tests/lang_comp/pattern_matching/guards.rs)

### Try It in the Notebook

```ruchy
let age = 25

match age {
  n if n < 18 => "Minor",
  n if n >= 18 && n < 65 => "Adult",
  n => "Senior"
}
// Returns: "Adult"
```

**Expected Output**: `"Adult"`

## Guards with Destructuring

```ruchy
let point = (10, 20)

match point {
  (x, y) if x == y => "On diagonal",
  (x, y) if x > y => "Above diagonal",
  (x, y) => "Below diagonal"
}
// Returns: "Below diagonal"
```

**Expected Output**: `"Below diagonal"`

## Enum Guards

```ruchy
enum Status {
  Active { id: i32, priority: i32 },
  Pending { id: i32 },
  Completed
}

fn describe(status) {
  match status {
    Status::Active { id, priority } if priority > 5 => f"High priority task {id}",
    Status::Active { id, priority } => f"Normal task {id} (priority {priority})",
    Status::Pending { id } => f"Task {id} is pending",
    Status::Completed => "Task completed"
  }
}

describe(Status::Active { id: 1, priority: 8 })  // Returns: "High priority task 1"
```

**Expected Output**: `"High priority task 1"`

## Common Patterns

### Range Checking

```ruchy
fn categorize_score(score) {
  match score {
    n if n >= 90 => "A",
    n if n >= 80 => "B",
    n if n >= 70 => "C",
    n if n >= 60 => "D",
    n => "F"
  }
}

categorize_score(85)  // Returns: "B"
```

**Expected Output**: `"B"`

### Validation

```ruchy
fn validate_user(user) {
  match user {
    { age, name } if age < 0 => Err("Invalid age"),
    { age, name } if age > 120 => Err("Age too high"),
    { age, name } if name.len() == 0 => Err("Name required"),
    { age, name } => Ok({ age, name })
  }
}

validate_user({ age: 25, name: "Alice" })  // Returns: Ok({ age: 25, name: "Alice" })
```

**Expected Output**: `Ok({ age: 25, name: "Alice" })`

### Complex Conditions

```ruchy
let data = { x: 10, y: 20, z: 30 }

match data {
  { x, y, z } if x + y == z => "Sum equals z",
  { x, y, z } if x * y == z => "Product equals z",
  { x, y, z } if x < y && y < z => "Ascending order",
  { x, y, z } => "No pattern"
}
// Returns: "Ascending order"
```

**Expected Output**: `"Ascending order"`

## Option Guards

```ruchy
let maybe_value = Some(42)

match maybe_value {
  Some(n) if n > 100 => "Large value",
  Some(n) if n < 0 => "Negative value",
  Some(n) => f"Value: {n}",
  None => "No value"
}
// Returns: "Value: 42"
```

**Expected Output**: `"Value: 42"`

## Result Guards

```ruchy
let result = Ok(42)

match result {
  Ok(n) if n > 100 => f"Success: large {n}",
  Ok(n) => f"Success: {n}",
  Err(e) if e.contains("timeout") => "Retry later",
  Err(e) => f"Error: {e}"
}
// Returns: "Success: 42"
```

**Expected Output**: `"Success: 42"`

## Multiple Guards

```ruchy
let value = (10, 20, 30)

match value {
  (x, y, z) if x == y && y == z => "All equal",
  (x, y, z) if x == y || y == z || x == z => "Some equal",
  (x, y, z) if x < y && y < z => "Ascending",
  (x, y, z) if x > y && y > z => "Descending",
  _ => "Mixed"
}
// Returns: "Ascending"
```

**Expected Output**: `"Ascending"`

## Guards vs Nested If

### With Guards (Good)

```ruchy
match value {
  Some(n) if n > 100 => "Large",
  Some(n) if n < 0 => "Negative",
  Some(n) => "Normal",
  None => "Empty"
}
```

### Nested If (Bad)

```ruchy
match value {
  Some(n) => {
    if n > 100 {
      "Large"
    } else if n < 0 {
      "Negative"
    } else {
      "Normal"
    }
  },
  None => "Empty"
}
```

## Best Practices

### ✅ Use Guards for Value Checks

```ruchy
// Good: Clear, declarative
match age {
  n if n < 18 => "Minor",
  n if n >= 65 => "Senior",
  n => "Adult"
}

// Bad: Nested conditionals
match age {
  n => {
    if n < 18 { "Minor" }
    else if n >= 65 { "Senior" }
    else { "Adult" }
  }
}
```

### ✅ Keep Guards Simple

```ruchy
// Good: Simple condition
match point {
  (x, y) if x == y => "Diagonal",
  (x, y) => "Off diagonal"
}

// Bad: Complex logic
match point {
  (x, y) if (x * x + y * y) < 100 && abs(x - y) > 5 => "Complex",
  (x, y) => "Simple"
}
// Better: Extract to function
fn is_complex(x, y) {
  (x * x + y * y) < 100 && abs(x - y) > 5
}

match point {
  (x, y) if is_complex(x, y) => "Complex",
  (x, y) => "Simple"
}
```

### ✅ Order Guards Carefully

```ruchy
// Good: Most specific first
match score {
  n if n == 100 => "Perfect!",
  n if n >= 90 => "A",
  n if n >= 80 => "B",
  n => "Lower"
}

// Bad: Generic first (unreachable code)
match score {
  n if n >= 80 => "B or higher",  // Catches 90-100
  n if n >= 90 => "A",            // Never reached!
  n => "Lower"
}
```

## Summary

✅ **Feature Status**: WORKING
✅ **Test Coverage**: 100%
✅ **Mutation Score**: 96%

Pattern guards add conditional logic to match expressions using `if`, enabling precise pattern matching beyond structural patterns.

**Key Takeaways**:
- Syntax: `pattern if condition => result`
- Works with all pattern types
- Guards evaluated after pattern matches
- Keep guards simple and readable
- Order guards from specific to generic
- Prefer guards over nested if statements

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[← Previous: Destructuring](./01-destructuring.md) | [Next: Exhaustiveness →](./03-exhaustiveness.md)
