実験的に Seq<'T> と Async<'T> を混ぜてみた

IO.fs

module IO =
    open System.IO
    open System.Threading.Tasks

    // 普通の unfold
    let readLines (reader : TextReader) =
        match reader with
        | null -> nullArg "reader"
        | r -> r |> Seq.unfold (fun r -> match r.ReadLine() with null -> None | s -> Some (s, r))

    // asyncReadLines : TextReader -> string seq 用のアクティブパターン
    let private (|Complete|Fault|) (task : 'a Task) =
        async {
            try
                let! r = task |> Async.AwaitTask
                return Complete r
            with ex -> return Fault ex // エラー発生のケースは未テスト
        } |> Async.RunSynchronously

    // Task<'T> を混ぜてみたパターン
    let asyncReadLines (reader : TextReader) =
        match reader with
        | null -> nullArg "reader"
        | r -> r |> Seq.unfold (fun r ->
            // (|Complete|Fault|) を利用して Task を即実行、成功の可否で分岐させる
            match r.ReadLineAsync() with
            | Fault _ -> None
            | Complete s -> if isNull s then None else Some (s, r))

IO.asyncReadLines stdin
|> Seq.iter (printfn "%s")