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# `impl Trait`

`impl Trait` can be used in two locations:

1. as an argument type
2. as a return type

## As an argument type

If your function is generic over a trait but you don't mind the specific type, you can simplify the function declaration using `impl Trait` as the type of the argument.

For example, consider the following code:

    fn parse_csv_document<R: std::io::BufRead>(src: R) -> std::io::Result<Vec<Vec<String>>> {
        src.lines()
            .map(|line| {
                // For each line in the source
                line.map(|line| {
                    // If the line was read successfully, process it, if not, return the error
                    line.split(',') // Split the line separated by commas
                        .map(|entry| String::from(entry.trim())) // Remove leading and trailing whitespace
                        .collect() // Collect all strings in a row into a Vec<String>
                })
            })
            .collect() // Collect all lines into a Vec<Vec<String>>
    }

`parse_csv_document` is generic, allowing it to take any type which implements BufRead, such as `BufReader<File>` or `[u8]`, but it's not important what type `R` is, and `R` is only used to declare the type of `src`, so the function can also be written as:

    fn parse_csv_document(src: impl std::io::BufRead) -> std::io::Result<Vec<Vec<String>>> {
        src.lines()
            .map(|line| {
                // For each line in the source
                line.map(|line| {
                    // If the line was read successfully, process it, if not, return the error
                    line.split(',') // Split the line separated by commas
                        .map(|entry| String::from(entry.trim())) // Remove leading and trailing whitespace
                        .collect() // Collect all strings in a row into a Vec<String>
                })
            })
            .collect() // Collect all lines into a Vec<Vec<String>>
    }

Note that using `impl Trait` as an argument type means that you cannot explicitly state what form of the function you use, i.e. `parse_csv_document::<std::io::Empty>(std::io::empty())` will not work with the second example.

## As a return type

If your function returns a type that implements `MyTrait`, you can write its return type as `-> impl MyTrait`. This can help simplify your type signatures quite a lot!

    use std::iter;
    use std::vec::IntoIter;

    // This function combines two `Vec<i32>` and returns an iterator over it.
    // Look how complicated its return type is!
    fn combine_vecs_explicit_return_type(
        v: Vec<i32>,
        u: Vec<i32>,
    ) -> iter::Cycle<iter::Chain<IntoIter<i32>, IntoIter<i32>>> {
        v.into_iter().chain(u.into_iter()).cycle()
    }

    // This is the exact same function, but its return type uses `impl Trait`.
    // Look how much simpler it is!
    fn combine_vecs(
        v: Vec<i32>,
        u: Vec<i32>,
    ) -> impl Iterator<Item=i32> {
        v.into_iter().chain(u.into_iter()).cycle()
    }

    fn main() {
        let v1 = vec![1, 2, 3];
        let v2 = vec![4, 5];
        let mut v3 = combine_vecs(v1, v2);
        assert_eq!(Some(1), v3.next());
        assert_eq!(Some(2), v3.next());
        assert_eq!(Some(3), v3.next());
        assert_eq!(Some(4), v3.next());
        assert_eq!(Some(5), v3.next());
        println!("all done");
    }

More importantly, some Rust types can't be written out. For example, every closure has its own unnamed concrete type. Before `impl Trait` syntax, you had to allocate on the heap in order to return a closure. But now you can do it all statically, like this:

    // Returns a function that adds `y` to its input
    fn make_adder_function(y: i32) -> impl Fn(i32) -> i32 {
        let closure = move |x: i32| { x + y };
        closure
    }

    fn main() {
        let plus_one = make_adder_function(1);
        assert_eq!(plus_one(2), 3);
    }

You can also use `impl Trait` to return an iterator that uses `map` or `filter` closures! This makes using `map` and `filter` easier. Because closure types don't have names, you can't write out an explicit return type if your function returns iterators with closures. But with `impl Trait` you can do this easily:

    fn double_positives<'a>(numbers: &'a Vec<i32>) -> impl Iterator<Item = i32> + 'a {
        numbers
            .iter()
            .filter(|x| x > &&0)
            .map(|x| x * 2)
    }

    fn main() {
        let singles = vec![-3, -2, 2, 3];
        let doubles = double_positives(&singles);
        assert_eq!(doubles.collect::<Vec<i32>>(), vec![4, 6]);
    }