My ghc(i) crashes when using STM data invariants. This little piece of code demonstrates the problem: module Bug where import Control.Concurrent.STM test = do x <- atomically $ do v <- newTVar 0 always $ return True -- remove this line and all is fine return v atomically (readTVar x) >>= print This is what ghci makes of it: ben@sarun> ghci Bug.hs GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Ok, modules loaded: Bug. *Bug> test Loading package syb ... linking ... done. Loading package array-0.2.0.0 ... linking ... done. Loading package stm-2.1.1.2 ... linking ... done. zsh: segmentation fault ghci Bug.hs I am using ghc-6.10.1 freshly installed from source with just a 'cabal install stm' thrown after it. BTW, the documentation for Control.Concurrent.STM.TVar lists... nothing. Similar with Control.Monad.STM. Well, at least the source link works, so one isn't completely lost... :-) Cheers Ben