second-largest-4.hs

import Data.Foldable

secondLargest :: (Ord a, Foldable t) => t a -> Maybe a
secondLargest = snd . foldl' secondLargest' (Nothing, Nothing) where
    secondLargest' (Nothing, Nothing) x = (Just x, Just x)
    secondLargest' (Just largest, Just second) x 
        | x > largest = (Just x, Just largest)
        | x > second  = (Just largest, Just x)
        | otherwise   = (Just largest, Just second)
    secondLargest' tuple _ = tuple