Thanks to Haskell's laziness, the following implementation of `minimum` is O(n):

  import Data.List (sortBy)

  minimum :: Ord a => [a] -> a
  minimum = head . sortBy compare

`sortBy` is O(n log n). But `head` only needs the first element which can be found in linear time. So `sortBy` does not compute the rest and `minimum` is O(n).