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 19-27, where we've defined a function
add_one that adds one 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.
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 19-27: 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 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 function to turn a vector of
numbers into a vector of strings, we could use a closure, like this:
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,
like this:
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 earlier
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.
Recall from the "Enum values" section of 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, like so:
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 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.
The following code tries to return a closure directly, but it won't compile:
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
|
help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
|
1 | fn returns_closure() -> impl Fn(i32) -> i32 {
| ~~~~
help: box the return type, and wrap all of the returned values in `Box::new`
|
1 ~ fn returns_closure() -> Box<dyn Fn(i32) -> i32> {
2 ~ Box::new(|x| x + 1)
|
For more information about this error, try `rustc --explain E0746`.
error: could not compile `functions-example` (lib) due to 1 previous error
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.
Next, let's look at macros!