Add fields from a directory tree with XML files using lenses, Directory Tree

SumXmlField.hs

#!/usr/bin/env stack
{-
stack
--resolver lts-4.2
--install-ghc runghc
--package xml-lens
--package directory-tree
-}
{-# LANGUAGE OverloadedStrings #-}
module Main where
import qualified System.Directory.Tree as DT
import System.FilePath ( takeExtension )
import System.Environment (getArgs)

import Text.XML
import Text.XML.Lens
import Text.Printf (printf)

import Data.Text as T
import Data.Text.Read (decimal)

main = do
    [diskRoot] <- getArgs -- Gross...
    tree <- DT.readDirectoryWith readTiles diskRoot
    printf "Total tiles: %d\n" (sum $ DT.dirTree tree)
  where
    readTiles :: FilePath -> IO Int
    readTiles f 
        | takeExtension f `elem` [".xml", ".XML"] = do
            doc <-  Text.XML.readFile def f
            let lens = root . el "isd" ./ el "TIL" ./ el "NUMTILES" . text 
            return $ either (const 0) fst $ decimal (doc ^. lens)
        | otherwise = return 0