Hello, Haskellers. I am working on HDBI and I faced with the problem. There is an error when I close SQlite database, "unable to close due to unfinalized statements or unfinished backups". This problems occurs when there is some not finalized statements related to this database. So, I must protect the user from this error and garantee the finalisation of all the statements BEFORE the disconnection from the database. There is naive implementation of weak child list inherited from HDBC https://github.com/s9gf4ult/hdbi/blob/master/Database/HDBI/DriverUtils.hs Here you can see that ChildList is just MVar to list of weak pointers. Every time when the statement is created the new weak pointer prepended to this list. Every time when statement becomes not reachable the weak pointer becomes empty and it's finalizer is scheduled to execute in parralel thread. The finalizer finishes the statement, so if finalizer is executed the statement is finished. Before the actual call of disconnection the 'disconnect' method performs 'closeAllChildren'. Call of 'closeAllChildren' performs finishing of just not empty weak pointers, because the finishing of not reachable statements is imposible. You can see the implementation here https://github.com/s9gf4ult/hdbi-sqlite/blob/master/Database/HDBI/SQlite/Imp... https://github.com/s9gf4ult/hdbi-sqlite/blob/master/Database/HDBI/SQlite/Imp... and here is the simplest code which cause an error {-# LANGUAGE OverloadedStrings #-} module Main where import System.Mem import Database.HDBI.SQlite import Database.HDBI perf c = do runRaw c "create table integers (val)" s <- prepare c "select * from integers" return () main = do c <- connectSqlite3 ":memory:" perf c performGC disconnect c If you remove 'performGC' error not occurs, because the weak pointer in ChildList is still not empty and 'finish' is performed in 'closeAllChildren'. The problem occures when the statement becomes not reachable and weak pointer becomes empty, but finalizer is not performed yet, because the finalizer performed in parralel thread according to https://www.google.ru/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CC8QFjAA&url=http%3A%2F%2Fcommunity.haskell.org%2F~simonmar%2Fpapers%2Fweak.pdf&ei=vRALUvrELaaS4ASH_oCQCw&usg=AFQjCNEWQtRfh5ei7J_Qd-VDVMq0ied0KQ&sig2=nsvzz4FXB_4dWp75CU6gzg The naive solution is to make 'closeAllChildren' to wait until all finalizers is completely performed. But how ? Maybe you have the better solution to garantee that all statements are finished before the database disconnecting ?