% Building Up

Alright, we'll start with building the list. That's pretty straight-forward with this new system. new is still trivial, just None out all the fields. Also because it's getting a bit unwieldy, let's break out a Node constructor too:

impl<T> Node<T> {
    fn new(elem: T) -> Rc<RefCell<Self>> {
        Rc::new(RefCell::new(Node {
            elem: elem,
            prev: None,
            next: None,
        }))
    }
}

impl<T> List<T> {
    pub fn new() -> Self {
        List { head: None, tail: None }
    }
}

> cargo build

**A BUNCH OF DEAD CODE WARNINGS BUT IT BUILT**

Yay!

Now let's try to write pushing onto the front of the list. Because doubly-linked lists are signficantly more complicated, we're going to need to do a fair bit more work. Where singly-linked list operations could be reduced to an easy one-liner, doubly-linked list ops are fairly complicated.

In particular we now need to specially handle some boundary cases around empty lists. Most operations will only touch the head or tail pointer. However when transitioning to or from the empty list, we need to edit both at once.

An easy way for us to validate if our methods make sense is if we maintain the following invariant: each node should have exactly two pointers to it. Each node in the middle of the list is pointed at by its predecessor and successor, while the nodes on the ends are pointed to by the list itself.

Let's take a crack at it:

pub fn push_front(&mut self, elem: T) {
    // new node needs +2 links, everything else should be +0
    let new_head = Node::new(elem);
    match self.head.take() {
        Some(old_head) => {
            // non-empty list, need to connect the old_head
            old_head.prev = Some(new_head.clone()); // +1 new_head
            new_head.next = Some(old_head);         // +1 old_head
            self.head = Some(new_head);             // +1 new_head, -1 old_head
            // total: +2 new_head, +0 old_head -- OK!
        }
        None => {
            // empty list, need to set the tail
            self.tail = Some(new_head.clone());     // +1 new_head
            self.head = Some(new_head);             // +1 new_head
            // total: +2 new_head -- OK!
        }
    }
}

cargo build
   Compiling lists v0.1.0 (file:///Users/ABeingessner/dev/too-many-lists/lists)
src/fourth.rs:37:17: 37:30 error: attempted access of field `prev` on type `alloc::rc::Rc<core::cell::RefCell<fourth::Node<T>>>`, but no field with that name was found
src/fourth.rs:37                 old_head.prev = Some(new_head.clone());
                                 ^~~~~~~~~~~~~
src/fourth.rs:38:17: 38:30 error: attempted access of field `next` on type `alloc::rc::Rc<core::cell::RefCell<fourth::Node<T>>>`, but no field with that name was found
src/fourth.rs:38                 new_head.next = Some(old_head);
                                 ^~~~~~~~~~~~~
error: aborting due to 2 previous errors
Could not compile `lists`.

Alright. Compiler error. Good start. Good start.

Why can't we access the prev and next fields on our nodes? It worked before when we just had an Rc<Node>. Seems like the RefCell is getting in the way.

We should probably check the docs.

Google's "rust refcell"

clicks first link

A mutable memory location with dynamically checked borrow rules

See the module-level documentation for more.

clicks link

Shareable mutable containers.

Values of the Cell<T> and RefCell<T> types may be mutated through shared references (i.e. the common &T type), whereas most Rust types can only be mutated through unique (&mut T) references. We say that Cell<T> and RefCell<T> provide 'interior mutability', in contrast with typical Rust types that exhibit 'inherited mutability'.

Cell types come in two flavors: Cell<T> and RefCell<T>. Cell<T> provides get and set methods that change the interior value with a single method call. Cell<T> though is only compatible with types that implement Copy. For other types, one must use the RefCell<T> type, acquiring a write lock before mutating.

RefCell<T> uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can claim temporary, exclusive, mutable access to the inner value. Borrows for RefCell<T>s are tracked 'at runtime', unlike Rust's native reference types which are entirely tracked statically, at compile time. Because RefCell<T> borrows are dynamic it is possible to attempt to borrow a value that is already mutably borrowed; when this happens it results in thread panic.

When to choose interior mutability

The more common inherited mutability, where one must have unique access to mutate a value, is one of the key language elements that enables Rust to reason strongly about pointer aliasing, statically preventing crash bugs. Because of that, inherited mutability is preferred, and interior mutability is something of a last resort. Since cell types enable mutation where it would otherwise be disallowed though, there are occasions when interior mutability might be appropriate, or even must be used, e.g.

• Introducing inherited mutability roots to shared types.
• Implementation details of logically-immutable methods.
• Mutating implementations of Clone.

Introducing inherited mutability roots to shared types

Shared smart pointer types, including Rc<T> and Arc<T>, provide containers that can be cloned and shared between multiple parties. Because the contained values may be multiply-aliased, they can only be borrowed as shared references, not mutable references. Without cells it would be impossible to mutate data inside of shared boxes at all!

It's very common then to put a RefCell<T> inside shared pointer types to reintroduce mutability:

use std::collections::HashMap;
use std::cell::RefCell;
use std::rc::Rc;

fn main() {
    let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
    shared_map.borrow_mut().insert("africa", 92388);
    shared_map.borrow_mut().insert("kyoto", 11837);
    shared_map.borrow_mut().insert("piccadilly", 11826);
    shared_map.borrow_mut().insert("marbles", 38);
}

Note that this example uses Rc<T> and not Arc<T>. RefCell<T>s are for single-threaded scenarios. Consider using Mutex<T> if you need shared mutability in a multi-threaded situation.

Hey, Rust's docs continue to be incredibly awesome.

The meaty bit we care about is this line:

shared_map.borrow_mut().insert("africa", 92388);

In particular, the borrow_mut thing. Seems we need to explicitly borrow a RefCell. The . operator's not going to do it for us. Weird. Let's try:

pub fn push_front(&mut self, elem: T) {
    let new_head = Node::new(elem);
    match self.head.take() {
        Some(old_head) => {
            old_head.borrow_mut().prev = Some(new_head.clone());
            new_head.borrow_mut().next = Some(old_head);
            self.head = Some(new_head);
        }
        None => {
            self.tail = Some(new_head.clone());
            self.head = Some(new_head);
        }
    }
}

> cargo build
   Compiling lists v0.1.0 (file:///Users/ABeingessner/dev/too-many-lists/lists)
src/fourth.rs:12:5: 12:12 warning: struct field is never used: `elem`, #[warn(dead_code)] on by default
src/fourth.rs:12     elem: T,
                     ^~~~~~~

Hey, it built! Docs win again.