Haskell Practice: List Sorted on Second Element of Tuple

practice_2019_04_11.hs

{- Write a program which returns a list of sorted tuples based on the
 - second element:
 - e.g. tuple_list = [ (a, 5), (b, 2), (c, 3), (d, 1), (e, 4) ]
 - Output: [d, b, c, e, a]
 -}

import Data.Tuple
import Data.List

foo :: (Ord b) => [(a, b)] -> [a]
foo xs = map fst sorted
  where sorted = sortOn snd xs