Git object hash in FSharp

git-hash-object.fs

let calcObjHash (objData:byte array) =
    use sha1 = new System.Security.Cryptography.SHA1Managed ()
    let pre = 
        sprintf "blob %d\000" objData.Length
        |> System.Text.Encoding.UTF8.GetBytes
    use ms = new System.IO.MemoryStream ()
    ms.Write (pre, 0, pre.Length)
    ms.Write (objData, 0, objData.Length)
    ms.Position <- 0L
    sha1.ComputeHash (ms)
    |> BitConverter.ToString 
    |> fun s -> s.Replace("-", "").ToLower ()