1. Open the Cargo.toml file that has been generated earlier for you.

2. In the bin folder, create a file called return_abstract.rs.

3. Add the following code, and run it with cargo run --bin return_abstract:

1   #![feature(conservative_impl_trait)]
2   
3   trait Animal {
4     fn do_sound(&self);
5   }
6   
7   struct Dog;
8   impl Animal for Dog {
9     fn do_sound(&self) {
10      println!("Woof");
11    }
12  }
13  
14  fn main() {
15    // The caller doesn't know which exact object he gets
16    // He knows only that it implements the Animal trait
17    let animal = create_animal();
18    animal.do_sound();
19  
20    for word in caps_words_iter("do you feel lucky, punk‽") {
21      println!("{}", word);
22    }
23  
24    let multiplier = create_multiplier(23);
25    let result = multiplier(3);
26    println!("23 * 3 = {}", result);
27  }
28  
29  // The impl trait syntax allows us to use abstract return types
30  // This means that we don't specify which exact struct we return
31  // but which trait(s) it implements
32  fn create_animal() -> impl Animal {
33    Dog {}
34  }
35  
36  // Any iterator can be returned as an abstract return type
37  fn caps_words_iter<'a>(text: &'a str) -> impl Iterator + 'a {
38    // Return an iterator over every word converted into ALL_CAPS
39    text.trim().split(' ').map(|word| word.to_uppercase())
40  }
41  
42  // Same goes for closures
43  fn create_multiplier(a: i32) -> impl Fn(i32) -> i32 {
44    move |b| a * b
45  }