X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/9f6c55ebcab2e1d3073e8bb6c8c910d0116efee4..d29f51c818176f4249179ceba33de1ff41db2c90:/src/part10.rs diff --git a/src/part10.rs b/src/part10.rs index 53fb0b3..2ba31f3 100644 --- a/src/part10.rs +++ b/src/part10.rs @@ -1,53 +1,176 @@ -// Rust-101, Part 09: Closures (WIP) -// ================================= - -use std::io::prelude::*; -use std::io; +// 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(&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); + 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); + 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(&self, mut a: A) { for digit in self { - a(digit); + // 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, 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(v: &Vec) { + //@ `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, divisor: i32) -> Vec { + //@ 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`, and later +// learned about `Option` in Part 04. `Option` 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](part08.html) | [next](main.html) +//@ [index](main.html) | [previous](part09.html) | [raw source](workspace/src/part10.rs) | +//@ [next](part11.html)