Thanks for the responses. The semantics of check as described the "Data Invariants" paper on STM are really close to what I am looking for. Unfortunately, that work is not in GHC yet, so I'm attempting my own poor substitute (below). I have a store that is to be accessed by other threads, and I want to throw an error under certain conditions. (The names below are borrowed from the "Scheme in 48 Hours" tutorial.) In reality I'm looking for an inconsistent store, but for simplicity's sake I've just put in a standard read operation below. Is there an easier way? Thanks, -Rod -- module Store where import Control.Monad.Error import Control.Concurrent.STM data StoreError = DoesNotExist String | Default String instance Error StoreError where noMsg = Default "Store error" strMsg = Default type ThrowsError = Either StoreError type IOThrowsError = ErrorT StoreError IO type Store a = TVar [ (String, TVar a) ] -- read from the store getSTM :: String -> Store a -> STM (ThrowsError a) getSTM name store = do st <- readTVar store case lookup name st of Nothing -> return (Left $ DoesNotExist name) Just rd -> readTVar rd >>= return . Right get :: String -> Store a -> IOThrowsError a get name store = do d <- liftIO $ atomically $ getSTM name store case d of Left err -> throwError err Right d' -> return d'