option_asyncmap.rs

use std::future::Future;

use futures::future::{self, Either, FutureExt, Map, Ready};

trait AsyncOption<T> {
  fn async_map<U, Fun, Fut>(
    self,
    f: Fun,
  ) -> Either<Map<Fut, &'static dyn Fn(Fut::Output) -> Option<U>>, Ready<Option<U>>>
  where
    Fun: FnOnce(T) -> Fut,
    Fut: Future<Output = U>;
}

impl<T> AsyncOption<T> for Option<T> {
  fn async_map<U, Fun, Fut>(
    self,
    f: Fun,
  ) -> Either<Map<Fut, &'static dyn Fn(Fut::Output) -> Option<U>>, Ready<Option<U>>>
  where
    Fun: FnOnce(T) -> Fut,
    Fut: Future<Output = U>,
  {
    match self {
      Some(v) => Either::Left(f(v).map(&Some)),
      None => Either::Right(future::ready(None)),
    }
  }
}

#[tokio::main]
async fn main() {
  let result = Some(37i32).async_map(add_five).await;
  println!("{:?}", result);  // prints Some(42)
}

async fn add_five(x: i32) -> i32 {
  x + 5
}