Find first not repeated char in a string. "Technical question" -> h. "Backend" -> b

a.hs

-- f (x:xs) = case x `elem` xs of
--   True -> f xs
--   False -> x

f s = f' $ map toLower s
  where
    f' (x:xs) =
      case x `elem` xs of
        True -> f' xs
        False -> x

f s = f' $ map toLower s
  where f' (x:xs) = case x `elem` xs of True -> f' xs; False -> x

-- one liner version using if/else/then
f s = f' $ map toLower s where f' (x:xs) = if x `elem` xs then f' xs else x