RustBrock/Advanced Functions and Closures.md

Advanced Functions and Closures

This section includes some more advanced features including function pointers and returning closures.

Function Pointers

We have 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 that you have already defined rather than defining a new closure.

Functions coerce to the type fn (with a lowercase f), not to be confused with, not to be confused with the Fn closure trait.

The fn type is called a function pointer.

Passing 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 below.

This shows that we have defined a function add_one that adds one to its parameter.

The function do_twice takes two parameters: a function pointer to any function 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}");
}

Output

The answer is: 12

Here we specify that the parameter f in do_twice is an fn that takes one parameter of type i32 and returns a i32.

We can then call f from the body of do_twice.

In main we can pass the function name add_one as the first arg to do_twice.

Unlike closures, fn is a type rather than a trait, so we need to 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).

This means that you can always pass a function pointer as an argument for a function that expects a closure.

It is best to write functions using a generic type and one of the closure traits so your functions can accept either functions or closures.

That being said, one example of where you would only want to 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.

Lets look at a use of the map method provided by the Iterator trait in the std library.

To use the map function to turn a vector of numbers into a vector of strings, we could use a closure.

Like this:

    let list_of_numbers = vec![1, 2, 3];
    let list_of_strings: Vec<String> =
        list_of_numbers.iter().map(|i| i.to_string()).collect();

We could have a function as the argument to map instead of the closure.

Like this:

    let list_of_numbers = vec![1, 2, 3];
    let list_of_strings: Vec<String> =
        list_of_numbers.iter().map(ToString::to_string).collect();

Note, we must use the fully qualified syntax that we talked about earlier in "Advanced Traits" section.

This is because there are multiple functions available named to_string.

Here we are using the to_string function defined in the ToString trait, which is in the std library has implemented for any type that implements Display.

Recall form the "Enum values" section of Ch6 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, this means we can specify the initializer functions as arguments for methods that take closures.

Like this:

    enum Status {
        Value(u32),
        Stop,
    }

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

Here we creates Status::Value instances using each u32 value in the range that map is called on by using the initializer function of Status::Value.

Some prefer to use this style, and some people prefer to use closures.

They compile to the same code, so use whatever style is clearer to you.

Returning Closures