The Rust Programming Language

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

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.

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.

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!