Hi, I´m trying to make a searchable transactional cache using STM. The whole idea is to use indexed TVar variables using a FiniteMap. another TVar holds the finitemap . This last TVar has to be a global variable. I found that when handled as global, a TVar does not keep the state. For example: stmcache= newTVar 0 main= do v<- val v<- val print v val = atomically $! do cache <- stmcache x <- readTVar cache writeTVar cache (x+1) return x this code print 0 instead of 1 as the result of incrementing cache and retrieving it the second time The same happens with: iocache= atomically $ newTVar 0 `debug` "newTVar" main= do v<- val v<- val print v getChar val = do cache<- iocache atomically $! do x <- readTVar cache writeTVar cache (x+1) return x here i used a IO TVar global instead of a STM TVar This other version , that passes TVar as parameter, does work: iocache= atomically $ newTVar 0 `debug` "newTVar" main= do cache<-iocache v<- val cache v<- val cache print v val cache= atomically $! do x <- readTVar cache writeTVar cache (x+1) I need the global version since the cache acces may be in any location on the program.... Anyone know a solution using STM? The MVar version works OK, but blocking the entire index is not the best solution....