# Enum
1. 🌟🌟 Enums can be created with explicit discriminator.

```rust,editable

// fix the errors
enum Number {
    Zero,
    One,
    Two,
}

enum Number1 {
    Zero = 0,
    One,
    Two,
}

// C-like enum
enum Number2 {
    Zero = 0.0,
    One = 1.0,
    Two = 2.0,
}


fn main() {
    // a enum variant can be converted to a integer by `as`
    assert_eq!(Number::One, Number1::One);
    assert_eq!(Number1::One, Number2::One);

    println!("Success!")
} 
```

2. 🌟 each enum variant can hold its own data.
```rust,editable

// fill in the blank
enum Message {
    Quit,
    Move { x: i32, y: i32 },
    Write(String),
    ChangeColor(i32, i32, i32),
}

fn main() {
    let msg1 = Message::Move{__}; // instantiating with x = 1, y = 2 
    let msg2 = Message::Write(__); // instantiating with "hello, world!"

    println!("Success!")
} 
```

3. 🌟🌟 we can get the data which a enum variant is holding by pattern match
```rust,editable

// fill in the blank and fix the error
enum Message {
    Quit,
    Move { x: i32, y: i32 },
    Write(String),
    ChangeColor(i32, i32, i32),
}

fn main() {
    let msg = Message::Move{x: 1, y: 2};

    if let Message::Move{__} = msg {
        assert_eq!(a, b);
    } else {
        panic!("NEVER LET THIS RUN！");
    }

    println!("Success!")
} 
```

4. 🌟🌟 

```rust,editable

// fill in the blank and fix the errors
enum Message {
    Quit,
    Move { x: i32, y: i32 },
    Write(String),
    ChangeColor(i32, i32, i32),
}

fn main() {
    let msgs: __ = [
        Message::Quit,
        Message::Move{x:1, y:3},
        Message::ChangeColor(255,255,0)
    ];

    for msg in msgs {
        show_message(msg)
    }
} 

fn show_message(msg: Message) {
    println!("{}", msg);
}
```

5. 🌟🌟 As there is no `null` in Rust, we have to use enum  `Option<T>`  to deal the cases when value is absent.
```rust,editable

// fill in the blank to make the `println` work.
// also add some code to prevent the `panic` from running.
fn main() {
    let five = Some(5);
    let six = plus_one(five);
    let none = plus_one(None);

    if let __ = six {
        println!("{}", n);

        println!("Success!")
    } 
        
    panic!("NEVER LET THIS RUN！");
} 

fn plus_one(x: Option<i32>) -> Option<i32> {
    match x {
        __ => None,
        __ => Some(i + 1),
    }
}
```


6. 🌟🌟🌟🌟 implement a `linked-list` via enums.

```rust,editable

use crate::List::*;

enum List {
    // Cons: Tuple struct that wraps an element and a pointer to the next node
    Cons(u32, Box<List>),
    // Nil: A node that signifies the end of the linked list
    Nil,
}

// Methods can be attached to an enum
impl List {
    // Create an empty list
    fn new() -> List {
        // `Nil` has type `List`
        Nil
    }

    // Consume a list, and return the same list with a new element at its front
    fn prepend(self, elem: u32) -> __ {
        // `Cons` also has type List
        Cons(elem, Box::new(self))
    }

    // Return the length of the list
    fn len(&self) -> u32 {
        // `self` has to be matched, because the behavior of this method
        // depends on the variant of `self`
        // `self` has type `&List`, and `*self` has type `List`, matching on a
        // concrete type `T` is preferred over a match on a reference `&T`
        // after Rust 2018 you can use self here and tail (with no ref) below as well,
        // rust will infer &s and ref tail. 
        // See https://doc.rust-lang.org/edition-guide/rust-2018/ownership-and-lifetimes/default-match-bindings.html
        match *self {
            // Can't take ownership of the tail, because `self` is borrowed;
            // instead take a reference to the tail
            Cons(_, ref tail) => 1 + tail.len(),
            // Base Case: An empty list has zero length
            Nil => 0
        }
    }

    // Return representation of the list as a (heap allocated) string
    fn stringify(&self) -> String {
        match *self {
            Cons(head, __ tail) => {
                // `format!` is similar to `print!`, but returns a heap
                // allocated string instead of printing to the console
                format!("{}, {}", head, tail.__())
            },
            Nil => {
                format!("Nil")
            },
        }
    }
}

fn main() {
    // Create an empty linked list
    let mut list = List::new();

    // Prepend some elements
    list = list.prepend(1);
    list = list.prepend(2);
    list = list.prepend(3);

    // Show the final state of the list
    println!("linked list has length: {}", list.len());
    println!("{}", list.stringify());
}
```

> You can find the solutions [here](https://github.com/sunface/rust-by-practice)(under the solutions path), but only use it when you need it