auto merge of #16408 : steveklabnik/rust/guide_iterators, r=brson

An introduction to iterators. I kinda like this, but I kinda don't. Hmmm.

Author: bors

src/doc/guide.md

@@ -4227,9 +4227,343 @@ Doing this is not particularly common, but every once in a while, it's useful.
 That's all you need to get the hang of closures! Closures are a little bit
 strange at first, but once you're used to using them, you'll miss them in any
 language that doesn't have them. Passing functions to other functions is
-incredibly powerful.  Next, let's look at one of those things: iterators.
+incredibly powerful. Next, let's look at one of those things: iterators.
 
-# iterators
+# Iterators
+
+Let's talk about loops.
+
+Remember Rust's `for` loop? Here's an example:
+
+```{rust}
+for x in range(0i, 10i) {
+    println!("{:d}", x);
+}
+```
+
+Now that you know more Rust, we can talk in detail about how this works. The
+`range` function returns an **iterator**. An iterator is something that we can
+call the `.next()` method on repeatedly, and it gives us a sequence of things.
+
+Like this:
+
+```{rust}
+let mut range = range(0i, 10i);
+
+loop {
+    match range.next() {
+        Some(x) => {
+            println!("{}", x);
+        }
+        None => { break }
+    }
+}
+```
+
+We make a mutable binding to the return value of `range`, which is our iterator.
+We then `loop`, with an inner `match`. This `match` is used on the result of
+`range.next()`, which gives us a reference to the next value of the iterator.
+`next` returns an `Option<int>`, in this case, which will be `Some(int)` when
+we have a value and `None` once we run out. If we get `Some(int)`, we print it
+out, and if we get `None`, we `break` out of the loop.
+
+This code sample is basically the same as our `for` loop version. The `for`
+loop is just a handy way to write this `loop`/`match`/`break` construct.
+
+`for` loops aren't the only thing that uses iterators, however. Writing your
+own iterator involves implementing the `Iterator` trait. While doing that is
+outside of the scope of this guide, Rust provides a number of useful iterators
+to accomplish various tasks. Before we talk about those, we should talk about a
+Rust anti-pattern. And that's `range`.
+
+Yes, we just talked about how `range` is cool. But `range` is also very
+primitive. For example, if you needed to iterate over the contents of
+a vector, you may be tempted to write this:
+
+```{rust}
+let nums = vec![1i, 2i, 3i];
+
+for i in range(0u, nums.len()) {
+    println!("{}", nums[i]);
+}
+```
+
+This is strictly worse than using an actual iterator. The `.iter()` method on
+vectors returns an iterator which iterates through a reference to each element
+of the vector in turn. So write this:
+
+```{rust}
+let nums = vec![1i, 2i, 3i];
+
+for num in nums.iter() {
+    println!("{}", num);
+}
+```
+
+There are two reasons for this. First, this is more semantic. We iterate
+through the entire vector, rather than iterating through indexes, and then
+indexing the vector. Second, this version is more efficient: the first version
+will have extra bounds checking because it used indexing, `nums[i]`. But since
+we yield a reference to each element of the vector in turn with the iterator,
+there's no bounds checking in the second example. This is very common with
+iterators: we can ignore unnecessary bounds checks, but still know that we're
+safe.
+
+There's another detail here that's not 100% clear because of how `println!`
+works. `num` is actually of type `&int`, that is, it's a reference to an `int`,
+not an `int` itself. `println!` handles the dereferencing for us, so we don't
+see it. This code works fine too:
+
+```{rust}
+let nums = vec![1i, 2i, 3i];
+
+for num in nums.iter() {
+    println!("{}", *num);
+}
+```
+
+Now we're explicitly dereferencing `num`. Why does `iter()` give us references?
+Well, if it gave us the data itself, we would have to be its owner, which would
+involve making a copy of the data and giving us the copy. With references,
+we're just borrowing a reference to the data, and so it's just passing
+a reference, without needing to do the copy.
+
+So, now that we've established that `range` is often not what you want, let's
+talk about what you do want instead.
+
+There are three broad classes of things that are relevant here: iterators,
+**iterator adapters**, and **consumers**. Here's some definitions:
+
+* 'iterators' give you a sequence of values.
+* 'iterator adapters' operate on an iterator, producing a new iterator with a
+  different output sequence.
+* 'consumers' operate on an iterator, producing some final set of values.
+
+Let's talk about consumers first, since you've already seen an iterator,
+`range`.
+
+## Consumers
+
+A 'consumer' operates on an iterator, returning some kind of value or values.
+The most common consumer is `collect()`. This code doesn't quite compile,
+but it shows the intention:
+
+```{rust,ignore}
+let one_to_one_hundred = range(0i, 100i).collect();
+```
+
+As you can see, we call `collect()` on our iterator. `collect()` takes
+as many values as the iterator will give it, and returns a collection 
+of the results. So why won't this compile? Rust can't determine what
+type of things you want to collect, and so you need to let it know.
+Here's the version that does compile:
+
+```{rust}
+let one_to_one_hundred = range(0i, 100i).collect::<Vec<int>>();
+```
+
+If you remember, the `::<>` syntax allows us to give a type hint,
+and so we tell it that we want a vector of integers.
+
+`collect()` is the most common consumer, but there are others too. `find()`
+is one:
+
+```{rust}
+let one_to_one_hundred = range(0i, 100i);
+
+let greater_than_forty_two = range(0i, 100i)
+                             .find(|x| *x >= 42);
+
+match greater_than_forty_two {
+    Some(_) => println!("We got some numbers!"),
+    None    => println!("No numbers found :("),
+}
+```
+
+`find` takes a closure, and works on a reference to each element of an
+iterator. This closure returns `true` if the element is the element we're
+looking for, and `false` otherwise. Because we might not find a matching
+element, `find` returns an `Option` rather than the element itself.
+
+Another important consumer is `fold`. Here's what it looks like:
+
+```{rust}
+let sum = range(1i, 100i)
+              .fold(0i, |sum, x| sum + x);
+```
+
+`fold()` is a consumer that looks like this:
+`fold(base, |accumulator, element| ...)`. It takes two arguments: the first
+is an element called the "base". The second is a closure that itself takes two
+arguments: the first is called the "accumulator," and the second is an
+"element." Upon each iteration, the closure is called, and the result is the
+value of the accumulator on the next iteration. On the first iteration, the
+base is the value of the accumulator.
+
+Okay, that's a bit confusing. Let's examine the values of all of these things
+in this iterator:
+
+| base | accumulator | element | closure result |
+|------|-------------|---------|----------------|
+| 0i   | 0i          | 1i      | 1i             |
+| 0i   | 1i          | 2i      | 3i             |
+| 0i   | 3i          | 3i      | 6i             |
+
+We called `fold()` with these arguments:
+
+```{rust}
+# range(1i, 5i)
+.fold(0i, |sum, x| sum + x);
+```
+
+So, `0i` is our base, `sum` is our accumulator, and `x` is our element.  On the
+first iteration, we set `sum` to `0i`, and `x` is the first element of `nums`,
+`1i`. We then add `sum` and `x`, which gives us `0i + 1i = 1i`. On the second
+iteration, that value becomes our accumulator, `sum`, and the element is
+the second element of the array, `2i`. `1i + 2i = 3i`, and so that becomes
+the value of the accumulator for the last iteration. On that iteration,
+`x` is the last element, `3i`, and `3i + 3i = 6i`, which is our final
+result for our sum. `1 + 2 + 3 = 6`, and that's the result we got.
+
+Whew. `fold` can be a bit strange the first few times you see it, but once it
+clicks, you can use it all over the place. Any time you have a list of things,
+and you want a single result, `fold` is appropriate.
+
+Consumers are important due to one additional property of iterators we haven't
+talked about yet: laziness. Let's talk some more about iterators, and you'll
+see why consumers matter.
+
+## Iterators
+
+As we've said before, an iterator is something that we can call the `.next()`
+method on repeatedly, and it gives us a sequence of things. Because you need
+to call the method, this means that iterators are **lazy**. This code, for
+example, does not actually generate the numbers `1-100`, and just creates a
+value that represents the sequence:
+
+```{rust}
+let nums = range(1i, 100i);
+```
+
+Since we didn't do anything with the range, it didn't generate the sequence.
+Once we add the consumer:
+
+```{rust}
+let nums = range(1i, 100i).collect::<Vec<int>>();
+```
+
+Now, `collect()` will require that `range()` give it some numbers, and so
+it will do the work of generating the sequence.
+
+`range` is one of two basic iterators that you'll see. The other is `iter()`,
+which you've used before. `iter()` can turn a vector into a simple iterator
+that gives you each element in turn:
+
+```{rust}
+let nums = [1i, 2i, 3i];
+
+for num in nums.iter() {
+   println!("{}", num);
+}
+```
+
+These two basic iterators should serve you well. There are some more
+advanced iterators, including ones that are infinite. Like `count`:
+
+```{rust}
+std::iter::count(1i, 5i);
+```
+
+This iterator counts up from one, adding five each time. It will give
+you a new integer every time, forever. Well, technically, until the
+maximum number that an `int` can represent. But since iterators are lazy,
+that's okay! You probably don't want to use `collect()` on it, though...
+
+That's enough about iterators. Iterator adapters are the last concept
+we need to talk about with regards to iterators. Let's get to it!
+
+## Iterator adapters
+
+"Iterator adapters" take an iterator and modify it somehow, producing
+a new iterator. The simplest one is called `map`:
+
+```{rust,ignore}
+range(1i, 100i).map(|x| x + 1i);
+```
+
+`map` is called upon another iterator, and produces a new iterator where each
+element reference has the closure it's been given as an argument called on it.
+So this would give us the numbers from `2-101`. Well, almost! If you
+compile the example, you'll get a warning:
+
+```{notrust,ignore}
+2:37 warning: unused result which must be used: iterator adaptors are lazy and
+              do nothing unless consumed, #[warn(unused_must_use)] on by default
+ range(1i, 100i).map(|x| x + 1i);
+ ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+```
+
+Laziness strikes again! That closure will never execute. This example
+doesn't print any numbers:
+
+```{rust,ignore}
+range(1i, 100i).map(|x| println!("{}", x));
+```
+
+If you are trying to execute a closure on an iterator for its side effects,
+just use `for` instead.
+
+There are tons of interesting iterator adapters. `take(n)` will get the
+first `n` items out of an iterator, and return them as a list. Let's
+try it out with our infinite iterator from before, `count()`:
+
+```{rust}
+for i in std::iter::count(1i, 5i).take(5) {
+    println!("{}", i);
+}
+```
+
+This will print
+
+```{notrust,ignore}
+1
+6
+11
+16
+21
+```
+
+`filter()` is an adapter that takes a closure as an argument. This closure
+returns `true` or `false`. The new iterator `filter()` produces returns
+only the elements that that closure returned `true` for:
+
+```{rust}
+for i in range(1i, 100i).filter(|x| x % 2 == 0) {
+    println!("{}", i);
+}
+```
+
+This will print all of the even numbers between one and a hundred.
+
+You can chain all three things together: start with an iterator, adapt it
+a few times, and then consume the result. Check it out:
+
+```{rust}
+range(1i, 1000i)
+    .filter(|x| x % 2 == 0)
+    .filter(|x| x % 3 == 0)
+    .take(5)
+    .collect::<Vec<int>>();
+```
+
+This will give you a vector containing `6`, `12`, `18`, `24`, and `30`.
+
+This is just a small taste of what iterators, iterator adapters, and consumers
+can help you with. There are a number of really useful iterators, and you can
+write your own as well. Iterators provide a safe, efficient way to manipulate
+all kinds of lists. They're a little unusual at first, but if you play with
+them, you'll get hooked. For a full list of the different iterators and
+consumers, check out the [iterator module documentation](std/iter/index.html).
 
 # Generics