haskell code to clean up whitespace, replaces tabs with spaces and deletes trailing whitespace

CleanWhite.hs

-- |strip trailing whitespace, and turn tabs into spaces
-- Note: using Text as its append performance beats String
cleanupWhiteSpace :: FilePath -> IO T.Text
cleanupWhiteSpace file = do
  contents <- T.readFile file
  let fixed = map cleanupOneLine (T.lines contents)
  return (T.unlines fixed)

tabWidth :: Int
tabWidth = 8

cleanupOneLine :: T.Text -> T.Text
cleanupOneLine str = str'
  where
    numSpacesForTab n = tabWidth - (n `mod` tabWidth)
    -- loop over the line, keeping current pos. Where a tab is found, insert
    -- spaces until the next tab stop. Discard any trailing whitespace.
    go col res cur =
      if T.null cur
         then res
         else
           case T.head cur of
             '\t' -> go (col + toAdd) (res <> T.replicate toAdd " ") (T.tail cur)
                where
                  toAdd = numSpacesForTab col
             c -> go (col + 1) (T.snoc res c) (T.tail cur)
    str1 = go 0 T.empty str
    str' = T.dropWhileEnd isSpace str1