KeepEqual.hs

{-| Keep equal elements that are at the same position in both lists

  Examples:

  >>> keepEqual "hello" "world"
  "l"

  >>> keepEqual (repeat 1) [0..10]
  [1]

  >>> keepEqual [True, False, True] (repeat True)
  [True,True]
-}

import Data.Maybe(catMaybes)

keepEqual :: Eq a => [a] -> [a] -> [a]
keepEqual xs ys = catMaybes $ zipWith match xs ys
 where
  match x y = if x == y then Just x else Nothing

main :: IO ()
main = do
  print $ keepEqual "hello" "world"
  print $ keepEqual (repeat 1) ([0..10] :: [Int])
  print $ keepEqual [True, False, True] (repeat True)