corrected.lua

require 'paths'

local current_dir = 'torch7'
checkFile = function( current_dir )
   local list = paths.dir( current_dir )
   table.sort(list, function (a,b) return a<b end)
    for i = 3, #list do
       list[i] = paths.concat(current_dir, list[i])
       if paths.filep( list[i] )then
          print( ' ==> found file '.. list[i] )
        else
           print( '==> check sub directory ' .. list[i])
       end
    end
end

checkFile(current_dir);

For someone new to paths tips are here