public/subjects/adding_twice/README.md

## adding_twice

### Instructions

You'll need to reuse your `add_curry` function. Copy and paste it directly into your `lib.rs` file.

Now create a function named `twice` using closures. This function will take a function `f(x)` as parameter, and return a function `f(f(x))`.

So, the purpose of this function is to add two times the value in `add_curry` to the original value.

### Expected functions

The type of the arguments are missing. Use the example `main` function to determine the correct type.

```rust
pub fn twice<T>(F: _) -> _{

}
```

### Usage

Here is a program to test your function.

```rust
use adding_twice::*;

fn main() {
    let add10 = add_curry(10);
    let value = twice(add10);
    println!("The value is {}", value(7));

    let add20 = add_curry(20);
    let value = twice(add20);
    println!("The value is {}", value(7));

    let add30 = add_curry(30);
    let value = twice(add30);
    println!("The value is {}", value(7));

    let neg = add_curry(-32);
    let value = twice(neg);
    println!("The value is {}", value(7));
}
```

And its output

```console
$ cargo run
The value is 27
The value is 47
The value is 67
The value is -57
$
```

### Notions

- [higher order function](https://doc.rust-lang.org/rust-by-example/fn/hof.html#higher-order-functions)