element

{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, OverlappingInstances #-}
import Data.Foldable (Foldable)
import qualified Data.Foldable as F
 
class Elem e c where
  element :: e -> c -> Bool
instance (Eq e, Foldable t) => Elem e (t e) where
  element = F.elem
instance (Elem e c, Foldable t) => Elem e (t c) where
  element e = F.any (element e)
 
main = do
  print $ (element :: Int -> [Int] -> Bool) 4 [1,2,3] -- False
  print $ (element :: Int -> [[Int]] -> Bool) 4 [[1],[2,4],[3]] -- True
  print $ (element :: [Int] -> [[Int]] -> Bool) [2] [[1],[2,4],[3]] -- False