x.cs

if (args.Length == 2 && args[1] == "-f")
{
    sha1(File.ReadAllBytes(args[2]));
}
else if (args.Length == 1)
{
    sha1(Encoding.ASCII.GetBytes(args[1]));
}
else if (args.Length == 0)
{
    sha1(Encoding.ASCII.GetBytes(Console.ReadToEnd());
}
else
{
    throw new Exception("error");
}

y.fs

match args with
| [| _; "-f"; path |] -> sha1(File.ReadAllBytes(path))
| [| _; arg |] -> sha1(Encoding.ASCII.GetBytes(arg))
| [| _ |] -> sha1(Encoding.ASCII.GetBytes(Console.ReadToEnd())
| _ -> failwith("error")

z.fs

match args with
| [| _; "-f"; path |] -> path |> File.ReadAllBytes |> sha1
| [| _; arg |] -> arg |> Encoding.ASCII.GetBytes |> sha1
| [| _ |] -> Console.ReadToEnd() |> Encoding.ASCII.GetBytes |> sha1
| _ -> failwith("error")