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

gem.hs

import Data.Set as S
import Data.List as L

process :: String -> String
process input =
  let lines' = lines input
      sets = L.map S.fromList lines'
      gemElements = L.foldl1 S.intersection sets
  in show $ S.size gemElements

main :: IO ()
main = interact process