{{
  {type KVStore} is an ability that models a key-value store.

  # Example

  @typecheck```
  KVStore.modify : (v -> v) -> k ->{KVStore k v} ()
  KVStore.modify f k = 
    KVStore.put (f KVStore.get)
  ```

  ```
  KVStore.modify (x -> x + 1)
  ```
}}
unique ability KVStore a b where
  get : a ->{KVStore a b} Optional b
  put : a -> b ->{KVStore a b} ()

{{ Adds a couple of key-value pairs to the {KVStore} and
   Reads them back. }}
myProgram : '{KVStore Nat Nat} Text
myProgram _ =
  use KVStore put
  put 3 4
  put 5 6
  maybeFour = KVStore.get 3
  Optional.map Nat.toText maybeFour
    |> Optional.getOrElse "nothing here"

{{ An in-memory key-value store that implements the {type KVStore} ability.
   
   ``inMemory p`` runs the computation ''p'', starting with an empty {type Map}
   as the store.
}}
inMemory : '{g, KVStore a b} r ->{g} r
inMemory keyValueInteractions =
  impl : Map a b -> abilities.Request {KVStore a b} r -> r
  impl map = cases
    { pure }                          -> pure
    {KVStore.get key -> resume}       ->
      handle resume (Map.get key map) with impl map
    {KVStore.put key value -> resume} ->
      updatedMap = Map.insert key value map
      handle !resume with impl updatedMap
  handle !keyValueInteractions with impl Map.empty