X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/369875f931d841112dd2b6651fc968bb6c569cdb..801f2b59728fba1e13d3e34a08457b812f8c0f56:/src/part09.rs diff --git a/src/part09.rs b/src/part09.rs index 023a48f..9cad15d 100644 --- a/src/part09.rs +++ b/src/part09.rs @@ -1,4 +1,172 @@ // 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`, 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.
+//@ `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 { + // 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!
+//@ 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)