1HaskellADay: keepEqual

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]
-}
keepEqual :: Eq a => [a] -> [a] -> [a]
keepEqual a b = map fst $ (filter $ uncurry (==)) $ zip a b

-- With more curry, just like Freddy.
keepEqual' :: Eq a => [a] -> [a] -> [a]
keepEqual' = curry $ map fst . (filter $ uncurry (==)) . uncurry zip