## Advanced Functions & Closures

Finally, let’s discuss some advanced features related to functions and closures: function pointers, diverging functions, and returning closures.

### Function Pointers

We’ve talked about how to pass closures to functions; you can also pass regular functions to functions! This is useful when we want to pass a function we’ve already defined rather than defining a new closure. We do this using function pointers to allow us to use functions as arguments to other functions. Functions coerce to the type `fn`, with a lower case ‘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 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 we can always pass a function pointer as an argument for a function that expects a closure. Prefer to write functions using a generic type and one of the closure traits, so that your functions can accept either functions or closures.

An example of a case where you’d 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.

For an example where we can 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:

    # #![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:

    # #![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 do have to use the fully qualified syntax that we talked about 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`.

Some people prefer this style, some people prefer to use closures. They end up with the same code, so use whichever feels more clear to you.

### Returning Closures

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

This code that tries to return a closure directly won’t compile:

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

The compiler error is:

    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

Our 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 in the previous section: we can use a trait object:

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

This code will compile just fine. For more about trait objects, refer back to the “Trait Objects” section in Chapter 17.

## Summary

Whew! Now we’ve gone over features of Rust that aren’t used often, but are available if you need them in very particular circumstances. We’ve introduced a lot of complex topics so that, when you encounter them in error message suggestions or in others’ code, you’ll at least have seen these concepts and syntax once before. You can use this chapter as a reference to guide you to your solutions.

Now, let’s put everything we’ve learned throughout the book into practice with one more project!