[rust-101.git] / src / part10.rs

// Rust-101, Part 10: Closures
// ===========================

use std::fmt;
use part05::BigInt;

//@ Assume we want to write a function that does *something* on, say, every digit of a `BigInt`.
//@ We will then need a way to express the action that we want to be taken, and to pass this to
//@ our function. In Rust, a natural first attempt to express this is to have a trait for it.

// So, let us define a trait that demands that the type provides some method `do_action` on digits.
//@ This immediately raises the question: How do we pass `self` to that function? Owned, shared
//@ reference, or mutable reference? The typical strategy to answer this question is to use the
//@ strongest type that still works. Certainly, passing `self` in owned form does not work: Then
//@ the function would consume `self`, and we could not call it again, on the second digit. So
//@ let's go with a mutable reference.
trait Action {
    fn do_action(&mut self, digit: u64);
}

// Now we can write a function that takes some `a` of a type `A` such that we can call `do_action`
// on `a`, passing it every digit.
impl BigInt {
    fn act_v1<A: Action>(&self, mut a: A) {
        //@ Remember that the `mut` above is just an annotation to Rust, telling it that we're okay
        //@ with `a` being mutated. Calling `do_action` on `a` takes a mutable reference, so
        //@ mutation could indeed happen.
        for digit in self {
            a.do_action(digit);                                     /*@*/
        }
    }
}

//@ As the next step, we need to come up with some action, and write an appropriate implementation
//@ of `Action` for it. So, let's say we want to print every digit, and to make this less boring,
//@ we want the digits to be prefixed by some arbitrary string. `do_action` has to know this
//@ string, so we store it in `self`.
struct PrintWithString {
    prefix: String,
}

impl Action for PrintWithString {
    // Here we perform the actual printing of the prefix and the digit. We're not making use of our
    // ability to change `self` here, but we could replace the prefix if we wanted.
    fn do_action(&mut self, digit: u64) {
        println!("{}{}", self.prefix, digit);                       /*@*/
    }
}

// Finally, this function takes a `BigInt` and a prefix, and prints the digits with the given prefix.
//@ It does so by creating an instance of `PrintWithString`, giving it the prefix, and then passing
//@ that to `act_v1`. Since `PrintWithString` implements `Action`, Rust now knows what to do.
fn print_with_prefix_v1(b: &BigInt, prefix: String) {
    let my_action = PrintWithString { prefix: prefix };
    b.act_v1(my_action);
}

// Here's a small main function, demonstrating the code above in action. Remember to edit `main.rs`
// to run it.
pub fn main() {
    let bignum = BigInt::new(1 << 63) + BigInt::new(1 << 16) + BigInt::new(1 << 63);
    print_with_prefix_v1(&bignum, "Digit: ".to_string());
}

// ## Closures
//@ Now, as it turns out, this pattern of describing some form of an action, that can carry in
//@ additional data, is very common. In general, this is called a *closure*. Closures take some
//@ arguments and produce a result, and they have an *environment* they can use, which corresponds
//@ to the type `PrintWithString` (or any other type implementing `Action`).
//@ Again we have the choice of passing this environment in owned or borrowed form, so there are
//@ three traits for closures in Rust: `Fn`-closures get a shared reference, `FnMut`-closures get a
//@ mutable reference, and `FnOnce`-closures consume their environment (and can hence be called
//@ only once). The syntax for a closure trait which takes arguments of type `T1`, `T2`, ... and
//@ returns something of type `U` is `Fn(T1, T2, ...) -> U`.

// This defines `act` very similar to above, but now we demand `A` to be the type of a closure that
// mutates its borrowed environment, takes a digit, and returns nothing.
impl BigInt {
    fn act<A: FnMut(u64)>(&self, mut a: A) {
        for digit in self {
            // We can call closures as if they were functions - but really, what's happening here
            // is translated to essentially what we wrote above, in `act_v1`.
            a(digit);                                               /*@*/
        }
    }
}

// Now that we saw how to write a function that operates on closures, let's see how to write a
// closure.
pub fn print_with_prefix(b: &BigInt, prefix: String) {
    //@ The syntax for closures is `|arg1, arg2, ...| code`. Notice that the closure can reference
    //@ variables like `prefix` that it did not take as argument - variables that happen to be
    //@ present *outside* of the closure. We say that the closure *captures* variables.
    //@ Rust will now automatically create a type (like `PrintWithString`) for the environment of
    //@ the closure with fields for every captured variable, implement the closure trait for this
    //@ type such that the action performed is given by the code of the closure, and finally it
    //@ will instantiate the environment type here at the definition site of the closure and fill
    //@ it appropriately.
    b.act(|digit| println!("{}{}", prefix, digit) );
}
// You can change `main` to call this function, and you should notice - nothing, no difference in
// behavior. But we wrote much less boilerplate code!

// Remember that we decided to use the `FnMut` trait above? This means our closure could actually
// mutate its environment. For example, we can use that to count the digits as they are printed.
pub fn print_and_count(b: &BigInt) {
    let mut count: usize = 0;
    //@ This time, the environment will contain a field of type `&mut usize`, that will be
    //@ initialized with a mutable reference of `count`. The closure, since it mutably borrows its
    //@ environment, is able to access this field and mutate `count` through it. Once `act`
    //@ returns, the closure is destroyed and `count` is no longer borrowed.
    //@ Because closures compile down to normal types, all the borrow checking continues to work as
    //@ usually, and we cannot accidentally leak a closure somewhere that still contains, in its
    //@ environment, a dead reference.
    b.act(|digit| { println!("{}: {}", count, digit); count = count +1; } );
    println!("There are {} digits", count);
}

// ## Fun with iterators and closures
//@ If you are familiar with functional languages, you are probably aware that one can have lots of
//@ fun with iterators and closures. Rust provides a whole lot of methods on iterators that allow
//@ us to write pretty functional-style list manipulation.

// Let's say we want to write a function that increments every entry of a `Vec` by some number,
// then looks for numbers larger than some threshold, and prints them.
fn inc_print_threshold(v: &Vec<i32>, offset: i32, threshold: i32) {
    //@ `map` takes a closure that is applied to every element of the iterator. `filter` removes
    //@ elements from the iterator that do not pass the test given by the closure.
    //@ 
    //@ Since all these closures compile down to the pattern described above, there is actually no
    //@ heap allocation going on here. This makes closures very efficient, and it makes
    //@ optimization fairly trivial: The resulting code will look like you hand-rolled the loop in
    //@ C.
    for i in v.iter().map(|n| *n + offset).filter(|n| *n > threshold) {
        println!("{}", i);
    }
}

// Sometimes it is useful to know both the position of some element in a list, and its value.
// That's where the `enumerate` function helps.
fn print_enumerated<T: fmt::Display>(v: &Vec<T>) {
    //@ `enumerate` turns an iterator over `T` into an iterator over `(usize, T)`, where the first
    //@ element just counts the position in the iterator. We can do pattern matching right in the
    //@ loop header to obtain names for both the position, and the value.
    for (i, t) in v.iter().enumerate() {
        println!("Position {}: {}", i, t);
    }
}

// And as a final example, one can also collect all elements of an iterator, and put them, e.g., in a vector.
fn filter_vec_by_divisor(v: &Vec<i32>, divisor: i32) -> Vec<i32> {
    //@ Here, the return type of `collect` is inferred based on the return type of our function. In
    //@ general, it can return anything implementing
    //@ [`FromIterator`](https://doc.rust-lang.org/stable/std/iter/trait.FromIterator.html).
    //@ Notice that `iter` gives us an iterator over borrowed `i32`, but we want to own them for
    //@ the result, so we insert a `map` to dereference.
    v.iter().map(|n| *n).filter(|n| *n % divisor == 0).collect()    /*@*/
}

// **Exercise 10.1**: Look up the
// [documentation of `Iterator`](https://doc.rust-lang.org/stable/std/iter/trait.Iterator.html)
// to learn about more functions that can act on iterators. Try using some of them. What about a
// function that sums the even numbers of an iterator? Or a function that computes the product of
// those numbers that sit at odd positions? A function that checks whether a vector contains a
// certain number? Whether all numbers are smaller than some threshold? Be creative!

// **Exercise 10.2**: We started the journey in Part 02 with `SomethingOrNothing<T>`, and later
// learned about `Option<T>` in Part 04. `Option<T>` also has a `map` function.
// [Read its documentation here.](https://doc.rust-lang.org/stable/std/option/enum.Option.html#method.map)
// Which functions in previous parts can you rewrite to use `map` instead?
// (Hint: read the source code of `map`, and see if the pattern appears in your own code.)
// (Bonus: [`test_invariant` in Part 05](part05.html#section-6) doesn't use `match`,
// but can you still find a way to rewrite it with `map`?)

//@ [index](main.html) | [previous](part09.html) | [raw source](workspace/src/part10.rs) |
//@ [next](part11.html)