Using F# active patterns to avoid boolean blindness.

cp_r.fs

let (|File|Directory|Missing|) path =
    if Directory.Exists path then Directory
    elif File.Exists path then File
    else Missing

let cp_r source dest =
    match source, dest with
    | Missing, _ -> failwithf "missing file"
    | File, File -> cp source dest
    | File, Directory -> ...
    | Directory, Directory -> ...