## Advanced Functions and Closures

Finally, we’ll explore some advanced features related to functions and closures, which include function pointers and returning closures.

### Function Pointers

We’ve talked about how to pass closures to functions; you can also pass regular functions to functions! This technique is useful when you want to pass a function you’ve already defined rather than defining a new closure. Doing this with function pointers will allow you to use functions as arguments to other functions. Functions coerce to the type `fn` (with a lowercase f), not to be confused with the `Fn` closure trait. The `fn` type is called a function pointer. The syntax for specifying that a parameter is a function pointer is similar to that of closures, as shown in Listing 19-35.

Filename: src/main.rs

    fn add_one(x: i32) -> i32 {
        x + 1
    }

    fn do_twice(f: fn(i32) -> i32, arg: i32) -> i32 {
        f(arg) + f(arg)
    }

    fn main() {
        let answer = do_twice(add_one, 5);

        println!("The answer is: {}", answer);
    }

Listing 19-35: Using the `fn` type to accept a function pointer as an argument

This code prints `The answer is: 12`. We specify that the parameter `f` in `do_twice` is an `fn` that takes one parameter of type `i32` and returns an `i32`. We can then call `f` in the body of `do_twice`. In `main`, we can pass the function name `add_one` as the first argument to `do_twice`.

Unlike closures, `fn` is a type rather than a trait, so we specify `fn` as the parameter type directly rather than declaring a generic type parameter with one of the `Fn` traits as a trait bound.

Function pointers implement all three of the closure traits (`Fn`, `FnMut`, and `FnOnce`), so you can always pass a function pointer as an argument for a function that expects a closure. It’s best to write functions using a generic type and one of the closure traits so your functions can accept either functions or closures.

An example of where you would want to only accept `fn` and not closures is when interfacing with external code that doesn’t have closures: C functions can accept functions as arguments, but C doesn’t have closures.

As an example of where you could use either a closure defined inline or a named function, let’s look at a use of `map`. To use the `map` function to turn a vector of numbers into a vector of strings, we could use a closure, like this:

    # #![allow(unused_variables)]
    #fn main() {
    let list_of_numbers = vec![1, 2, 3];
    let list_of_strings: Vec<String> = list_of_numbers
        .iter()
        .map(|i| i.to_string())
        .collect();
    #}

Or we could name a function as the argument to `map` instead of the closure, like this:

    # #![allow(unused_variables)]
    #fn main() {
    let list_of_numbers = vec![1, 2, 3];
    let list_of_strings: Vec<String> = list_of_numbers
        .iter()
        .map(ToString::to_string)
        .collect();
    #}

Note that we must use the fully qualified syntax that we talked about earlier in the “Advanced Traits” section because there are multiple functions available named `to_string`. Here, we’re using the `to_string` function defined in the `ToString` trait, which the standard library has implemented for any type that implements `Display`.

Another useful pattern exploits an implementation detail of tuple structs and tuple-struct enum variants. These items use `()` as initialiser syntax, which looks like a function call, and they’re actually implemented as functions returning an instance constructed from their arguments. They can also be called as a function pointer implementing the closure traits, and so can be used similarly to the above:

    # #![allow(unused_variables)]
    #fn main() {
    enum Status {
        Value(u32),
        Stop,
    }

    let list_of_statuses: Vec<Status> =
        (0u32..20)
        .map(Status::Value)
        .collect();
    #}

Some people prefer this style, and some people prefer to use closures. They end up compiling to the same code, so use whichever style is clearer to you.

### Returning Closures

Closures are represented by traits, which means you can’t return closures directly. In most cases where you might want to return a trait, you can instead use the concrete type that implements the trait as the return value of the function. But you can’t do that with closures because they don’t have a concrete type that is returnable; you’re not allowed to use the function pointer `fn` as a return type, for example.

The following code tries to return a closure directly, but it won’t compile:

    fn returns_closure() -> Fn(i32) -> i32 {
        |x| x + 1
    }

The compiler error is as follows:

    error[E0277]: the trait bound `std::ops::Fn(i32) -> i32 + 'static:
    std::marker::Sized` is not satisfied
     -->
      |
    1 | fn returns_closure() -> Fn(i32) -> i32 {
      |                         ^^^^^^^^^^^^^^ `std::ops::Fn(i32) -> i32 + 'static`
      does not have a constant size known at compile-time
      |
      = help: the trait `std::marker::Sized` is not implemented for
      `std::ops::Fn(i32) -> i32 + 'static`
      = note: the return type of a function must have a statically known size

The error references the `Sized` trait again! Rust doesn’t know how much space it will need to store the closure. We saw a solution to this problem earlier. We can use a trait object:

    # #![allow(unused_variables)]
    #fn main() {
    fn returns_closure() -> Box<dyn Fn(i32) -> i32> {
        Box::new(|x| x + 1)
    }
    #}

This code will compile just fine. For more about trait objects, refer to the “Using Trait Objects That Allow for Values of Different Types” section in Chapter 17.

Next, let’s look at macros!