Simple in-process key/value cache for Haskell

Cache.hs

-- | Simple in-process key/value cache
--
-- Of course, for really simple stuff you could probably use unsafeInterleaveIO
module Cache (Cache, newCache, fromCache) where

import Control.Monad (void)

-- Could also use STM instead
import Control.Concurrent (forkIO, Chan, newChan, readChan, writeChan, MVar, newEmptyMVar, putMVar, takeMVar)

import qualified Data.Map as Map

data CacheMsg k v = CacheMsg k (IO v) (MVar v)

-- | A single cache
newtype Cache k v = Cache (Chan (CacheMsg k v))

cacheThread :: (Ord k) => Chan (CacheMsg k v) -> IO ()
cacheThread chan = next Map.empty
	where
	next m = readChan chan >>= go m

	go m (CacheMsg k io reply) = case Map.lookup k m of
		Just v -> putMVar reply v >> next m
		Nothing -> do
			v <- io
			putMVar reply v
			next (Map.insert k v m)

-- | Create a new cache
newCache :: (Ord k) => IO (Cache k v)
newCache = do
	chan <- newChan
	-- This cache thread never terminates, so this is for a process-life cache
	-- That would be easy to change, but I won't bother here
	void $ forkIO (cacheThread chan)
	return (Cache chan)

syncCall :: Chan a -> (MVar r -> a) -> IO r
syncCall chan msg = do
	r <- newEmptyMVar
	writeChan chan (msg r)
	takeMVar r

-- | Get a value from the cache, or compute it and cache it
fromCache :: Cache k v -> k -> IO v -> IO v
fromCache (Cache chan) k io = syncCall chan (CacheMsg k io)

Yes, this design means only one operation can happen at a time, and that includes reads.