// Rust-101, Part 09: Iterators
// ============================
-// [index](main.html) | [previous](part08.html) | [next](main.html)
+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)
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