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

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

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

1   #![feature(conservative_impl_trait)]
2   
3   // The compose! macro takes a variadic amount of closures and  
    returns
4   // a closure that applies them all one after another
5   macro_rules! compose {
6     ( $last:expr ) => { $last };
7     ( $head:expr, $ ($tail:expr), +) => {
8       compose_two($head, compose!($ ($tail), +))
9     };
10  }
11  
12  // compose_two is a helper function used to
13  // compose only two closures into one
14  fn compose_two<FunOne, FunTwo, Input, Intermediate, Output>(
15    fun_one: FunOne,
16    fun_two: FunTwo,
17  ) -> impl Fn(Input) -> Output
18  where
19    FunOne: Fn(Input) -> Intermediate,
20    FunTwo: Fn(Intermediate) -> Output,
21  {
22    move |x| fun_two(fun_one(x))
23  }
24  
25  fn main() {
26    let add = |x| x + 2.0;
27    let multiply = |x| x * 3.0;
28    let divide = |x| x / 4.0;
29    // itermediate(x) returns ((x + 2) * 3) / 4
30    let intermediate = compose!(add, multiply, divide);
31  
32    let subtract = |x| x - 5.0;
33    // finally(x) returns (((x + 2) * 3) / 4) - 5
34    let finally = compose!(intermediate, subtract);
35  
36    println!("(((10 + 2) * 3) / 4) - 5 is: {}", finally(10.0));
37  }