[Haskell-cafe] acid state

[Anatoly Yakovenko]

So I am trying to understand how acid state works.  The HelloWorld
example has a

type Message = String
data Database = Database [Message]

$(deriveSafeCopy 0 'base ''Database)

-- Transactions are defined to run in either the 'Update' monad
-- or the 'Query' monad.
addMessage :: Message -> Update Database ()
addMessage msg
    = do Database messages <- get
         put $ Database (msg:messages)


It seems to me that since the Dababase is a list of messages every
update would require acid-state to rewrite the list into the file, so
each update would get slower as the list gets bigger, but what I am
seeing is that updates are constant time regardless of the size of the
list.  So how does it work?