X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/29958c0fd33c5e714b52bed79a1832113c43b8d8..9ae2b045dd1772c02f7013953dd4108a99bd2c74:/workspace/src/part09.rs

diff --git a/workspace/src/part09.rs b/workspace/src/part09.rs
index dc329bc..6b3f737 100644
--- a/workspace/src/part09.rs
+++ b/workspace/src/part09.rs
@@ -1,4 +1,123 @@
 // 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.
+            unimplemented!()
+        } else {
+            // Decrement, and return next digit.
+            unimplemented!()
+        }
+    }
+}
+
+// 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 {
+        unimplemented!()
+    }
+}
+
+// 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.
+
 
-// [index](main.html) | [previous](part08.html) | [next](main.html)