//@ 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); /*@*/
}
//@ 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`.
// 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);
}