Async<'a> list -> Async<'a list>

let sequence list =
    let rec loop l acc =
        async {
            match l with
            | [] ->
                return List.rev acc
            | h::t ->
                let! h' = h
                return! loop t (h'::acc)
        }
    loop list []

let sequenceList list =
    List.foldBack (map2 cons) list (retn [])