Fix link to lifetime elision rules

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

// Rust-101, Part 09: Iterators
// ============================

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 certain traits
//@ again. In particular, an iterator is something that implements the `Iterator` trait. As you can
//@ see in the [documentation](https://doc.rust-lang.org/stable/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, so it takes a reference.

//@ In writing this down, we again have to be explicit about the lifetime of the reference: We
//@ can't just have an `Iter`, we must have an `Iter<'a>` that borrows the number for lifetime
//@ `'a`. This is our first example of a data-type 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.
pub 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, nothing to do.
            None                                                    /*@*/
        } else {
            // Otherwise: 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 references, 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)
    }
}

// **Exercise 09.1**: Write a testcase for the iterator, making sure it yields the corrects numbers.
// 
// **Exercise 09.2**: Write a function `iter_ldf` that iterates over the digits with the
// least-significant digits coming first. Write a testcase for it.

// ## 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 reference 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. Rust enforces that `'a` spans every call to `next`, which
//@ means it has to span the loop.
//@ Thus `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, think of Java), which Rust successfully prevents.

// ## Iterator conversion trait
//@ If you closely compare the `for` loop in `main` above, with the one in `part06::vec_min`, you
//@ will notice that we were able to write `for e in v` earlier, but now we have to call `iter`.
//@ Why is that? Well, the `for` sugar is not actually tied to `Iterator`. Instead, it demands an
//@ implementation of
//@ [`IntoIterator`](https://doc.rust-lang.org/stable/std/iter/trait.IntoIterator.html).
//@ That's a trait of types that provide a *conversion* function into some kind of iterator. These
//@ conversion traits are a frequent pattern in Rust: Rather than demanding that something is an
//@ iterator, or a string, or whatever; one demands that something can be converted to an
//@ iterator/string/whatever. This provides convenience similar to overloading of functions: The
//@ function can be called with lots of different types.
//@ By implementing such traits for your types, you can even make your own types work smoothly with
//@ existing library functions. As usually for Rust, this abstraction comes at zero cost: If your
//@ data is already of the right type, the conversion function will not do anything and trivially
//@ be optimized away.

//@ If you have a look at the documentation of `IntoIterator`, you will notice that the function
//@ `into_iter` it provides actually consumes its argument. So we implement the trait for
//@ *references to* numbers, such that the number is not lost after the iteration.
impl<'a> IntoIterator for &'a BigInt {
    type Item = u64;
    type IntoIter = Iter<'a>;
    fn into_iter(self) -> Iter<'a> {
        self.iter()
    }
}
// With this in place, you can now replace `b.iter()` in `main` by `&b`. Go ahead and try it! <br/>
//@ Wait, `&b`? Why that? Well, we implemented `IntoIterator` for `&BigInt`. If we are in a place
//@ where `b` is already borrowed, we can just do `for digit in b`. If however, we own `b`, we have
//@ to create a reference to it. Alternatively, we could implement `IntoIterator` for `BigInt` -
//@ which, as already mentioned, would mean that `b` is actually consumed by the iteration, and
//@ gone.
//@ This can easily happen, for example, with a `Vec`: Both `Vec` and `&Vec` (and `&mut Vec`)
//@ implement `IntoIterator`, so if you do `for e in v`, and `v` has type `Vec`, then you will
//@ obtain ownership of the elements during the iteration - and destroy the vector in the process.
//@ We actually did that in `part01::vec_min`, but we did not care. You can write `for e in &v` or
//@ `for e in v.iter()` to avoid this.

//@ [index](main.html) | [previous](part08.html) | [raw source](workspace/src/part09.rs) |
//@ [next](part10.html)