let dirname = "c:\\"

let processFile filename = printfn "%s" filename
for filename in Directory.GetFiles(dirname) do processFile filename done

let processDirectory dirname proc =
let rec processDirectory' dirname' =
Directory.GetFiles(dirname') |> Array.iter proc
Directory.GetDirectories(dirname') |> Array.iter processDirectory'
processDirectory' dirname

// ------

let dirname = "c:\\"

processDirectory dirname (fun filename -> printfn "%s" filename)