The Rust Programming Language

Advanced Functions and Closures

This section explores some advanced features related to functions and closures, including 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. 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. Passing functions with function pointers will allow you to use functions as arguments to other functions.

The syntax for specifying that a parameter is a function pointer is similar to that of closures, as shown in Listing 20-28, where we’ve defined a function add_one that adds 1 to its parameter. The function do_twice takes two parameters: a function pointer to any function that takes an i32 parameter and returns an i32, and one i32 value. The do_twice function calls the function f twice, passing it the arg value, then adds the two function call results together. The main function calls do_twice with the arguments add_one and 5.

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}");
}

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), meaning 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.

That said, one 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 the map method provided by the Iterator trait in the standard library. To use the map method to turn a vector of numbers into a vector of strings, we could use a closure, as in Listing 20-29.

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. Listing 20-30 shows what this would look like.

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 in “Advanced Traits” 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.

Recall from “Enum values” in Chapter 6 that the name of each enum variant that we define also becomes an initializer function. We can use these initializer functions as function pointers that implement the closure traits, which means we can specify the initializer functions as arguments for methods that take closures, as seen in Listing 20-31.

fn main() {
    enum Status {
        Value(u32),
        Stop,
    }

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

Here we create Status::Value instances using each u32 value in the range that map is called on by using the initializer function of Status::Value. Some people prefer this style and some people prefer to use closures. They compile 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. However, you can’t usually 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 if the closure captures any values from its scope, for example.

Instead, you will normally use the impl Trait syntax we learned about in Chapter 10. You can return any function type, using Fn, FnOnce and FnMut. For example, the code in Listing 20-32 will work just fine.

#![allow(unused)]
fn main() {
fn returns_closure() -> impl Fn(i32) -> i32 {
    |x| x + 1
}
}

However, as we noted in “Closure Type Inference and Annotation” in Chapter 13, each closure is also its own distinct type. If you need to work with multiple functions that have the same signature but different implementations, you will need to use a trait object for them. Consider what happens if you write code like that shown in Listing 20-33.

fn main() {
    let handlers = vec![returns_closure(), returns_initialized_closure(123)];
    for handler in handlers {
        let output = handler(5);
        println!("{output}");
    }
}

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

fn returns_initialized_closure(init: i32) -> impl Fn(i32) -> i32 {
    move |x| x + init
}

Here we have two functions, returns_closure and returns_initialized_closure, which both return impl Fn(i32) -> i32. Notice that he closures that they return are different, even though they implement the same type. If we try to compile this, Rust lets us know that it won’t work:

$ cargo build
   Compiling functions-example v0.1.0 (file:///projects/functions-example)
    error[E0308]: mismatched types
    --> src/main.rs:4:9
    |
    4  |         returns_initialized_closure(123)
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected opaque type, found a different opaque type
    ...
    12 | fn returns_closure() -> impl Fn(i32) -> i32 {
    |                         ------------------- the expected opaque type
    ...
    16 | fn returns_initialized_closure(init: i32) -> impl Fn(i32) -> i32 {
    |                                              ------------------- the found opaque type
    |
    = note: expected opaque type `impl Fn(i32) -> i32` (opaque type at <src/main.rs:12:25>)
                found opaque type `impl Fn(i32) -> i32` (opaque type at <src/main.rs:16:46>)
    = note: distinct uses of `impl Trait` result in different opaque types

    For more information about this error, try `rustc --explain E0308`.
    error: could not compile `functions-example` (bin "functions-example") due to 1 previous error

The error message tells us that whenever we return an impl Trait Rust creates a unique opaque type, a type where we cannot see into the details of what Rust constructs for us. So even though these functions both return closures that implements the same trait, Fn(i32) -> i32, the opaque types Rust generates for each are distinct. (This is similar to how Rust produces different concrete types for distinct async blocks even when they have the same output type, as we saw in “Working with Any Number of Futures” in Chapter 17. We have seen a solution to this problem a few times now: we can use a trait object, as in Listing 20-34.

fn main() {
    let handlers = vec![returns_closure(), returns_initialized_closure(123)];
    for handler in handlers {
        let output = handler(5);
        println!("{output}");
    }
}

fn returns_closure() -> Box<dyn Fn(i32) -> i32> {
    Box::new(|x| x + 1)
}

fn returns_initialized_closure(init: i32) -> Box<dyn Fn(i32) -> i32> {
    Box::new(move |x| x + init)
}

This code will compile just fine. For more about trait objects, refer to the section “Using Trait Objects That Allow for Values of Different Types” in Chapter 18.

Next, let’s look at macros!