RecursiveCpDir.fsx

open System
open System.IO

module String =
    let split chars (s: string) = s.Split chars

/// Lazily returns every file and directory that has the given root path as a parent. Directories paths
/// occur before their children.
let rec getSystemEntriesRec path =
    seq { yield path
          for subPath in Directory.GetDirectories path do
            yield! getSystemEntriesRec subPath
          yield! Directory.GetFiles path }

let relativePath root fullPath =
    let sep = [|Path.DirectorySeparatorChar|]
    fullPath |> String.split sep |> Array.skip ((String.split sep root).Length) |> Path.Combine

let src, dst = "foo", "bar"

src
|> getSystemEntriesRec
|> Seq.map (fun srcFullPath ->
    srcFullPath, Path.Combine ([|dst; relativePath src srcFullPath|]))
|> Seq.iter (fun (src, dst) ->
    if File.Exists src then File.Copy (src, dst)
    else ignore (Directory.CreateDirectory dst))