Function map 

pub fn map<T, U, S, F>(s: S, f: F) -> impl Stream<Item = U>
where
    S: Stream<Item = T>,
    F: Fn(T) -> U,

Maps each value in a stream using a function.

Note

Prefer using the built-in StreamExt::map() method when chaining:

use futures::StreamExt;
let result = stream.map(|x| x * 2);

For async mappers, use StreamExt::then():

let result = stream.then(|x| async move { x * 2 });

This standalone function is provided for functional/pipe-style composition.