Data.HashMap-like structure, where each entity is implicitly indexed
by their last-modified time. Useful for keyed caches.
import qualified Data.TimeMap as TM import Control.Concurrent.STM main :: IO () main = do -- create a new, empty reference mapRef <- atomically TM.newTimeMap -- insert a new key/value pair in the map. Note that -- `someKey` should implement `Hashable`. This also -- sets the creation time of the value to "now". TM.insert someKey 0 mapRef -- adjusts the value of `someKey` to `1`. Note that -- also resets the creation time of the value to "now". TM.adjust (+1) someKey mapRef -- wait a second threadDelay 1000000 -- delete all values older than 1 second from now. atomically $ TM.filterFromNow 1 mapRef -- Will return `Nothing` atomically $ TM.lookup someKey mapRef return ()
How it works
There are two internal maps for a
TimeMap k a:
- a “time-indexed” map reference:
TVar (Map UTCTime (HashSet k))
- a mutable map of values:
STMContainers.Map.Map k (UTCTime, a)
Inserting a new value first performs a lookup on the hashtable to see if the key already exists:
- If it doesn’t, insert the current time and the element that into the mutable map. Then, insert the key as the value, and the time as the key into the time-indexed multimap.
- If it does, remove the entry from the time-indexed multimap for the old time, before doing the same thing as the first bullet.
This doesn’t have to create new data, and thus doesn’t need full
IO. All it does is
lookup the key in the mutable map.
To filter out all entries older than some time, you have to use the
function to split the time-indexed multimap into the entries you want to delete from
the main container, and the ones you want to keep. You simply
writeTVar the map you want
to keep, but for the ones you want to delete, you need to get all the
that map. Then, for every key that we need to delete, we delete it from the
main container. Suprisingly, this appears to operate in constant time, regardless of
the size of the map.
How to run tests
stack bench --benchmark-arguments="--output profile.html"
You can also view the results on my box here.