asyncOption.fs

type AsyncOptionBuilder() =
    member __.Return (value: 'T) : Async<Option<'T>> =
        async { return Some value }

    member __.ReturnFrom
        (asyncResult: Async<Option<_>>)
        : Async<Option<_>> =
        asyncResult

    member __.ReturnFrom
        (result: Option<_>)
        : Async<Option<_>> =
        async.Return result

    member __.Source(r: Result<'t, _>) = async { match r with Ok a -> return Some a | Error _ -> return None }
    member __.Source(a: Async<'t>) = async {
        let! inner = a
        return Some inner
    }

    member __.Bind
        (asyncResult: Async<Option<_>>, binder: 'T -> Async<Option<_>>)
        : Async<Option<_>> =
        async {
            let! result = asyncResult
            match result with
            | Some x -> return! binder x
            | None -> return None
        }

let asyncOption = new AsyncOptionBuilder()

async {
    // Example 1
    let expected = Some 42
    let! actual = asyncOption {
        let! value = async.Return(42)
        return value
    }
    printf "%A equals %A = %b" expected actual (expected = actual)

    // Example 2
    let expected = Some 42
    let! actual = asyncOption {
        let! value = async.Return(42)
        return value
    }
    printf "%A equals %A = %b" expected actual (expected = actual)
} |> Async.Start