The Rust Programming Language

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

We’ve talked about how to pass closures to functions, but you can pass regular functions to functions too! Functions coerce to the type fn, with a lower case ‘f’ not to be confused with the Fn closure trait. fn 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 19-38:

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 19-38: 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 when calling 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 only want to accept fn 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 example, if we wanted 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 the closure. They end up with the same code, so use whichever feels more clear to you.

Because closures are represented by traits, returning closures is a little tricky; we can’t do it directly. In most cases where we may want to return a trait, we can instead use the concrete type that implements the trait of what we’re returning as the return value of the function. We can’t do that with closures, though. 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
 --> <anon>:2:25
  |
2 | fn returns_closure() -> Fn(i32) -> i32 {
  |                         ^^^^^^^^^^^^^^ the trait `std::marker::Sized` is
  not implemented for `std::ops::Fn(i32) -> i32 + 'static`
  |
  = note: `std::ops::Fn(i32) -> i32 + 'static` does not have a constant size
  known at compile-time
  = note: the return type of a function must have a statically known size

The Sized trait again! Rust doesn’t know how much space it’ll need to store the closure. We saw a solution to this in the previous section, though: we can use a trait object:

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

For more about trait objects, refer back to Chapter 18.

Whew! Now we’ve gone over features of Rust that aren’t used very often, but are available if you need them. We’ve introduced a lot of complex topics so that when you encounter them in error message suggestions or when reading others’ code, you’ll at least have seen these concepts and syntax once before.

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