Copy and replace

// Inspired by : https://docs.microsoft.com/en-us/dotnet/standard/io/how-to-copy-directories
// Currently does NOT YET replace files (throws exception if file already exists, should instead remove existing file and overwrite, but be able to recover if something goes wrong during the process)

/// Copy and replace the contents from one directory to another. Including subdirectories.
let rec copyAndReplaceDirectory sourcePath targetPath : Result<unit list, string list> =
    let sourceDir = new DirectoryInfo(sourcePath)
    let targetDir = new DirectoryInfo(targetPath)

    match sourceDir.Exists, targetDir.Exists with
    | true, true ->
        // Directories exist, proceed with copy and replace

        // Copy all files
        for file in sourceDir.GetFiles() do
            file.CopyTo(Path.Combine(targetPath, file.Name)) |> ignore

        // Recursively copy subdirectories
        let subdirectoriesResults =
            [
                for subdirectory in sourceDir.GetDirectories() do
                    let subdirectoryTargetPath : string = Path.Combine(targetPath, subdirectory.Name)
                    copyAndReplaceDirectory subdirectory.FullName subdirectoryTargetPath
            ]

        subdirectoriesResults |> Result.sequence |> Result.mapError List.concat |> Result.map List.concat
    | true, false -> Error [ ("Failed copy and replace, target directory (" + targetPath + ") doesn't exist") ]
    | false, true -> Error [ ("Failed copy and replace, Source directory (" + sourcePath + ") doesn't exist") ]
    | false, false -> Error [ ("Failed copy and replace, target directory (" + targetPath + ") doesn't exist"); ("Failed copy and replace, Source directory (" + sourcePath + ") doesn't exist") ]