How do I transpose a Future out of an Option?

Question

Is there a more idiomatic or prettier way to do an operation like this in rust?

let maybe_output = match maybe_input {
    Some(input) => Some(async_result(input).await?),
    None => None,
};

I tried to use map like this,

let maybe_output = maybe_input.map(|input| async_result(input).await?);

but I cannot use the .await and ? operators inside a lambda that doesn't return a future or a result. I suspect I can do the map and have an Option<Future<Result<_>>>, then sequence out the Future<Result<_>> to be Future<Result<Option<_>>>;

let maybe_output = maybe_input.map(|input|
    async_result(input)
).transpose().await?;

but I don't know if I can call transpose on a Future since it's a trait not a type like Result or Option.

Answer 1

There is OptionFuture in the futures crate:

let maybe_output = OptionFuture::from(maybe_input.map(async_result)).await;

However, I would personally consider the original form more idiomatic. I can immediately understand what the match does, while I'd have to take a second look to know that OptionFuture does what I'd expect.