# Option Type - Feature 24/41

The Option type represents an optional value: either `Some(value)` or `None`. It eliminates null pointer errors by making absence explicit and type-safe.

## Option Definition

```ruchy
enum Option<T> {
  Some(T),
  None
}
```

**Test Coverage**: ✅ [tests/lang_comp/error_handling/option.rs](../../../../tests/lang_comp/error_handling/option.rs)

### Try It in the Notebook

```ruchy
let some_value = Some(42)
let no_value = None

some_value  // Returns: Some(42)
no_value    // Returns: None
```

**Expected Output**: `Some(42)`, `None`

## Creating Options

```ruchy
// Explicit construction
let name = Some("Alice")
let age = None

// From functions
fn find(arr, target) {
  for item in arr {
    if item == target {
      return Some(item)
    }
  }
  None
}

find([1, 2, 3], 2)  // Returns: Some(2)
find([1, 2, 3], 5)  // Returns: None
```

**Expected Output**: `Some(2)`, `None`

## Checking Option State

```ruchy
let value = Some(42)

value.is_some()  // Returns: true
value.is_none()  // Returns: false

let empty = None
empty.is_some()  // Returns: false
empty.is_none()  // Returns: true
```

**Expected Output**: `true`, `false`, `false`, `true`

## Unwrapping Values

### unwrap()

```ruchy
let value = Some(42)
value.unwrap()  // Returns: 42

let empty = None
empty.unwrap()  // Panics: "called unwrap() on None"
```

**Expected Output**: `42`, then panic

### unwrap_or()

```ruchy
let value = Some(42)
value.unwrap_or(0)  // Returns: 42

let empty = None
empty.unwrap_or(0)  // Returns: 0
```

**Expected Output**: `42`, `0`

### unwrap_or_else()

```ruchy
let value = Some(42)
value.unwrap_or_else(|| compute_default())  // Returns: 42

let empty = None
empty.unwrap_or_else(|| compute_default())  // Calls function
```

**Expected Output**: `42`, result of `compute_default()`

## Pattern Matching

```ruchy
let maybe_value = Some(42)

match maybe_value {
  Some(value) => f"Got {value}",
  None => "No value"
}
// Returns: "Got 42"
```

**Expected Output**: `"Got 42"`

### If Let

```ruchy
let result = Some(42)

if let Some(value) = result {
  f"Value: {value}"
} else {
  "No value"
}
// Returns: "Value: 42"
```

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

## Transforming Options

### map()

```ruchy
let value = Some(42)
value.map(|x| x * 2)  // Returns: Some(84)

let empty = None
empty.map(|x| x * 2)  // Returns: None
```

**Expected Output**: `Some(84)`, `None`

### and_then()

```ruchy
let value = Some(42)
value.and_then(|x| {
  if x > 0 {
    Some(x * 2)
  } else {
    None
  }
})
// Returns: Some(84)
```

**Expected Output**: `Some(84)`

### or()

```ruchy
let value = Some(42)
value.or(Some(0))  // Returns: Some(42)

let empty = None
empty.or(Some(0))  // Returns: Some(0)
```

**Expected Output**: `Some(42)`, `Some(0)`

## Common Patterns

### Safe Array Access

```ruchy
fn get(arr, index) {
  if index >= 0 && index < arr.len() {
    Some(arr[index])
  } else {
    None
  }
}

get([1, 2, 3], 1)   // Returns: Some(2)
get([1, 2, 3], 10)  // Returns: None
```

**Expected Output**: `Some(2)`, `None`

### Dictionary Lookup

```ruchy
let users = {
  "alice": { name: "Alice", age: 30 },
  "bob": { name: "Bob", age: 25 }
}

fn find_user(users, username) {
  if users.has_key(username) {
    Some(users[username])
  } else {
    None
  }
}

find_user(users, "alice")  // Returns: Some({ name: "Alice", age: 30 })
find_user(users, "charlie")  // Returns: None
```

**Expected Output**: `Some({ name: "Alice", age: 30 })`, `None`

### Null Coalescing

```ruchy
let config = {
  host: "localhost",
  port: None
}

let port = config.port.unwrap_or(8080)
port  // Returns: 8080
```

**Expected Output**: `8080`

### Chain Operations

```ruchy
fn parse_int(s) {
  // Returns Some(int) or None
}

fn double(n) {
  Some(n * 2)
}

let result = parse_int("42")
  .and_then(double)
  .unwrap_or(0)

result  // Returns: 84
```

**Expected Output**: `84`

## Option vs Null

| Feature | Option | Null |
|---------|--------|------|
| Type Safety | Explicit in signature | Hidden, runtime errors |
| Compiler Check | Forces handling | Silent propagation |
| Default Value | `unwrap_or(default)` | Manual checking |
| Chaining | `map`, `and_then` | Repeated null checks |
| Intent | Clear: may be absent | Ambiguous: forgot to set? |

```ruchy
// Option: Type-safe
fn find_user(id: i32) -> Option<User> {
  // ...
}

match find_user(123) {
  Some(user) => use_user(user),
  None => handle_not_found()
}

// Null: Unsafe
fn find_user(id: i32) -> User {
  // Returns null - crashes later!
}

let user = find_user(123)
user.name  // Crash if null!
```

## Best Practices

### ✅ Use Option for Optional Values

```ruchy
// Good: Clear that value may be absent
fn find(arr, target) -> Option<i32> {
  // ...
}

// Bad: -1 means not found? Ambiguous!
fn find(arr, target) -> i32 {
  // Returns -1 on not found
}
```

### ✅ Prefer unwrap_or over unwrap

```ruchy
// Good: Safe default
let port = config.port.unwrap_or(8080)

// Bad: Panics if None
let port = config.port.unwrap()
```

### ✅ Use Pattern Matching

```ruchy
// Good: Explicit handling
match maybe_value {
  Some(value) => process(value),
  None => use_default()
}

// Bad: Risky unwrap
let value = maybe_value.unwrap()
process(value)
```

### ✅ Chain with map and and_then

```ruchy
// Good: Functional, clear
result
  .map(|x| x * 2)
  .and_then(validate)
  .unwrap_or(default)

// Bad: Nested if-let
if let Some(x) = result {
  let doubled = x * 2
  if let Some(valid) = validate(doubled) {
    valid
  } else {
    default
  }
} else {
  default
}
```

## Summary

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

Option<T> represents optional values type-safely, eliminating null pointer errors. Use Some(value) for presence, None for absence, and handle both cases explicitly.

**Key Takeaways**:
- `Some(value)` vs `None`
- Check state: `is_some()`, `is_none()`
- Extract: `unwrap()`, `unwrap_or()`, `unwrap_or_else()`
- Transform: `map()`, `and_then()`, `or()`
- Pattern match for explicit handling
- Better than null: type-safe, compiler-checked

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