// Rust-101, Part 10: Closures
// ===========================
-use std::io::prelude::*;
-use std::{fmt,io};
+use std::fmt;
use part05::BigInt;
//@ Assume we want to write a function that does *something* on, say, every digit of a `BigInt`.
//@ 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 borrow,
-//@ or mutable borrow? The typical strategy to answer this question is to use the strongest
+//@ 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 borrow.
+//@ 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);
}
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 borrow, so mutation could indeed happen.
+ //@ 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); /*@*/
}
}
}
}
impl Action for PrintWithString {
- // Here we perform performs the actual printing of the prefix and the digit. We're not making use of our ability to
+ // 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); /*@*/
}
}
//@ 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 borrow, `FnMut`-closures get a mutable borrow, and `FnOnce`-closures consume their environment (and can hence
+//@ 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`.
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);
+ a(digit); /*@*/
}
}
}
// 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 borrow of
+ //@ 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 the borrow of `count` ends. Because closures compile down
+ //@ 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 borrow that has ended.
+ //@ that still contains, in its environment, a dead reference.
b.act(|digit| { println!("{}: {}", count, digit); count = count +1; } );
println!("There are {} digits", count);
}
//@
//@ 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) {
+ for i in v.iter().map(|n| *n + offset).filter(|n| *n > threshold) {
println!("{}", i);
}
}
// 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`](http://doc.rust-lang.org/stable/std/iter/trait.FromIterator.html).
- v.iter().filter(|n| *n % divisor == 0).collect()
+ //@ [`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`](http://doc.rust-lang.org/stable/std/iter/trait.Iterator.html) to learn about more functions
+// **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!
-//@ [index](main.html) | [previous](part08.html) | [next](main.html)
+//@ [index](main.html) | [previous](part09.html) | [raw source](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part10.rs) | [next](part11.html)
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