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 (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 19-27.

Filename: src/main.rs

Listing 19-27: 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 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.

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:

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

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

We have another useful pattern that exploits an implementation detail of tuple structs and tuple-struct enum variants. These types use () as initializer syntax, which looks like a function call. The initializers are actually implemented as functions returning an instance that’s constructed from their arguments. 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, like so:

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.

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.

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

The compiler error is as follows:

$ cargo build Compiling functions-example v0.1.0 (file:///projects/functions-example) error[E0746]: return type cannot have an unboxed trait object --> src/lib.rs:1:25 | 1 | fn returns_closure() -> dyn Fn(i32) -> i32 { | ^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time | = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits> help: use `impl Fn(i32) -> i32` as the return type, as all return paths are of type `[closure@src/lib.rs:2:5: 2:14]`, which implements `Fn(i32) -> i32` | 1 | fn returns_closure() -> impl Fn(i32) -> i32 { | ^^^^^^^^^^^^^^^^^^^ error: aborting due to previous error For more information about this error, try `rustc --explain E0746`. error: could not compile `functions-example` To learn more, run the command again with --verbose.

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:

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 section “Using Trait Objects That Allow for Values of Different Types” in Chapter 17.