part 09: explain how Rust prevents iterator invalidation

[rust-101.git] / src / part09.rs

// Rust-101, Part 09: Iterators (WIP)
// ==================================

use part05::BigInt;

// In the following, we will look into the iterator mechanism of Rust and make our `BigInt` compatible
// with the `for` loops. Of course, this is all about implementing particular traits again. In particular,
// an iterator is something that implements the `Iterator` trait. As you can see in [the documentation](http://doc.rust-lang.org/beta/std/iter/trait.Iterator.html),
// this trait mandates a single function `next` returning an `Option<Self::Item>`, where `Item` is an
// associated type chosen by the implementation. (There are many more methods provided for `Iterator`,
// but they all have default implementations, so we don't have to worry about them right now).
// 
// For the case of `BigInt`, we want our iterator to iterate over the digits in normal, notational order: The most-significant
// digit comes first. So, we have to write down some type, and implement `Iterator` for it such that `next` returns the digits
// one-by-one. Clearly, the iterator must somehow be able to access the number it iterates over, and it must store its current
// location. However, it cannot *own* the `BigInt`, because then the number would be gone after iteration! That'd certainly be bad.
// The only alternative is for the iterator to *borrow* the number.

// In writing this down, we again have to be explicit about the lifetime of the borrow: We can't just have an
// `Iter`, we must have an `Iter<'a>` that borrowed the number for lifetime `'a`. This is our first example of
// a datatype that's polymorphic in a lifetime, as opposed to a type. <br/>
// `usize` here is the type of unsigned, pointer-sized numbers. It is typically the type of "lengths of things",
// in particular, it is the type of the length of a `Vec` and hence the right type to store an offset into the vector of digits.
struct Iter<'a> {
    num: &'a BigInt,
    idx: usize, // the index of the last number that was returned
}

// Now we are equipped to implement `Iterator` for `Iter`.
impl<'a> Iterator for Iter<'a> {
    // We choose the type of things that we iterate over to be the type of digits, i.e., `u64`.
    type Item = u64;

    fn next(&mut self) -> Option<u64> {
        // First, check whether there's any more digits to return.
        if self.idx == 0 {
            // We already returned all the digits.
            None                                                    /*@*/
        } else {
            // Decrement, and return next digit.
            self.idx = self.idx - 1;                                /*@*/
            Some(self.num.data[self.idx])                           /*@*/
        }
    }
}

// All we need now is a function that creates such an iterator for a given `BigInt`.
impl BigInt {
    // Notice that when we write the type of `iter`, we don't actually have to give the lifetime parameter of `Iter`. Just as it is
    // the case with functions returning borrowed data, you can elide the lifetime. The rules for adding the lifetimes are exactly the
    // same. (See the last section of [part 06](part06.html).)
    fn iter(&self) -> Iter {
        Iter { num: self, idx: self.data.len() }                    /*@*/
    }
}

// We are finally ready to iterate! Remember to edit `main.rs` to run this function.
pub fn main() {
    let b = BigInt::new(1 << 63) + BigInt::new(1 << 16) + BigInt::new(1 << 63);
    for digit in b.iter() {
        println!("{}", digit);
    }
}

// Of course, we don't have to use `for` to apply the iterator. We can also explicitly call `next`.
fn print_digits_v1(b: &BigInt) {
    let mut iter = b.iter();
    // `loop` is the keyword for a loop without a condition: It runs endlessly, or until you break out of
    // it with `break` or `return`.
    loop {
        // Each time we go through the loop, we analyze the next element presented by the iterator - until it stops.
        match iter.next() {
            None => break,
            Some(digit) => println!("{}", digit)
        }
    }
}

// Now, it turns out that this combination of doing a loop and a pattern matching is fairly common, and Rust
// provides some convenient syntactic sugar for it.
fn print_digits_v2(b: &BigInt) {
    let mut iter = b.iter();
    // `while let` performs the given pattern matching on every round of the loop, and cancels the loop if the pattern
    // doesn't match. There's also `if let`, which works similar, but of course without the loopy part.
    while let Some(digit) = iter.next() {
        println!("{}", digit)
    }
}

// ## Iterator invalidation and lifetimes
// You may have been surprised that we had to explicitly annotate a lifetime when we wrote `Iter`. Of
// course, with lifetimes being present at every borrow in Rust, this is only consistent. But do we at
// least gain something from this extra annotation burden? (Thankfully, this burden only occurs when we
// define *types*, and not when we define functions - which is typically much more common.)
// 
// It turns out that the answer to this question is yes! This particular aspect of the concept of
// lifetimes helps Rust to eliminate the issue of *iterator invalidation*. Consider the following
// piece of code.
fn iter_invalidation_demo() {
    let mut b = BigInt::new(1 << 63) + BigInt::new(1 << 16) + BigInt::new(1 << 63);
    for digit in b.iter() {
        println!("{}", digit);
        /*b = b + BigInt::new(1);*/                                 /* BAD! */
    }
}
// If you enable the bad line, Rust will reject the code. Why? The problem is that we are modifying the
// number while iterating over it. In other languages, this can have all sorts of effects from inconsistent
// data or throwing an exception (Java) to bad pointers being dereferenced (C++). Rust, however, is able to
// detect this situation. When you call `iter`, you have to borrow `b` for some lifetime `'a`, and you obtain
// `Iter<'a>`. This is an iterator that's only valid for lifetime `'a`. Gladly, we have this annotation available
// to make such a statement. Now, since we are using the iterator throughout the loop, `'a` has to span the loop.
// This `b` is borrowed for the duration of the loop, and we cannot mutate it. This is yet another example for
// how the combination of mutation and aliasing leads to undesired effects (not necessarily crashes, like in Java),
// which Rust successfully prevents.
// 
// Technically speaking, there's one more subtlety that I did not explain yet. We never explicitly tied the lifetime `'a` of the
// iterator to the loop so how does this happen? The answer lies in the full type of `next()`:
// `fn<'a, 'b>(&'b mut Iter<'a>) -> Option<u64>`. Since `next()` takes a *borrowed* iterator, there are two lifetimes involved:
// The lifetime of the borrow of the iterator, and the lifetime of the iterator itself. In such a case of nested lifetimes,
// Rust implicitly adds the additional constraint that the inner lifetime *outlives* the outer one: The borrow of an iterator
// cannot be valid for longer than the iterator itself is valid. This means that the lifetime `'a` of the iterator needs
// to outlive every call to `next()`, and hence the loop. Lucky enough, this all happens without our intervention.


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