Haskell implementation of the C++ provided at https://medium.com/@nirjhor/functional-programming-in-c-8bdbd903ee32#.ehikr76vp

gem.hs

import Data.Set (fromList, intersection, size)
import Data.List (foldl1, map)

process :: String -> String
process = show . size . foldl1 intersection . map fromList . lines

main :: IO ()
main = interact process