On hashtables with "undo-able" insert

hashtable.hs

-- Just wrote down in the web editor.
-- Not tested at all but I hope you get the idea

import qualified Data.HashTable.ST.Basic as HT
import Data.List.NonEmpty(NonEmpty(..))
import qualified Data.List.NonEmpty as NE

type MyHashTable s k v = HT.HashTable s k (NonEmpty v)

insert :: MyHashTable s k v -> k -> v -> ST s ()
insert hashTable k v =
  HT.mutate hashTable k $ \prev ->
    (Just (v :| maybe [] NE.toList prev), ())

update :: MyHashTable s k v -> k -> v -> ST s ()
update hashTable k v = HT.insert hashTable k (v :| [])

pop :: MyHashTable s k v -> k -> ST s (Maybe v)
pop hashTable k =
  HT.mutate hashTable k $ \prev ->
    case prev of
      Nothing -> (Nothing, Nothing)
      Just (v :| rest) -> (NE.nonEmpty rest, Just v)