# Pastebin IeHn3hi9
import Data.Function ((&))

example :: [(String, [String])]
example =
  [ ("apple", ["crisp", "juicy", "floury"]),
    ("banana", ["fresh", "rotten"])
  ]

alistModify :: Eq k => k -> (Maybe v -> Maybe v) -> [(k, v)] -> [(k, v)]
alistModify key f [] = case f Nothing of
  Nothing -> []
  Just v -> [(key, v)]
alistModify key f ((k, v) : xs)
  | key == k = case f (Just v) of
    Nothing -> xs
    Just v' -> (k, v') : xs
  | otherwise = (k, v) : alistModify key f xs

example' :: [(String, [String])]
example' = example & alistModify "banana" f
  where f Nothing = Just ["tasty"]
        f (Just xs) = Just ("tasty":xs)