+import Data.Accessor
+import qualified Data.Accessor.Monad.Trans.State as MonadState
+import qualified Data.Map as Map
+import qualified Control.Monad as Monad
+import qualified Control.Monad.Trans.State as State
+
+-- Make a caching version of a stateful computatation.
+makeCached :: (Monad m, Ord k) =>
+ k -- ^ The key to use for the cache
+ -> Accessor s (Map.Map k v) -- ^ The accessor to get at the cache
+ -> State.StateT s m v -- ^ How to compute the value to cache?
+ -> State.StateT s m v -- ^ The resulting value, from the cache or freshly
+ -- computed.
+makeCached key accessor create = do
+ cache <- MonadState.get accessor
+ case Map.lookup key cache of
+ -- Found in cache, just return
+ Just value -> return value
+ -- Not found, compute it and put it in the cache
+ Nothing -> do
+ value <- create
+ MonadState.modify accessor (Map.insert key value)
+ return value