Hi, I'm trying to get MonadReader-like functionality in the IO monad. It doesn't appear possible implement it with the interfaces that Haskell98 or GHC provide. I'm looking for something like "thread-local variables". The interface could be something like this: newTLRef :: a -> IO (TLRef a) withTLRef :: TLRef a -> a -> IO b -> IO b readTLRef :: TLRef a -> IO a writeTLRef :: TLRef a -> a -> IO () modifyTLRef :: TLRef a -> (a -> a) -> IO () This would have a lot of uses. I am aware of the "Implicit Configurations" paper by Kiselyov and Shan, but a solution such as theirs which requires modifying the type signatures of all intermediate function calls is not suitable. I want to be able to say "run algorithm A using database D" without requiring all of the functions in algorithm A to know that databases are somehow involved. One way to look at it is that I am seeking something like the type-based approach, but easier and with less explicit syntax; another way to look at it is that I am seeking something like a global IORef based approach, but more safe. An implementation based on ThreadId-keyed maps is almost workable: data TLRef a = TLR a (MVar (Map ThreadId a)) The problem with this is that while it is possible to find out the ThreadId of the current thread, it doesn't appear to be possible to get the ThreadId of the parent thread, which would be needed for values to be properly inherited. Is there a way around this? Will there ever be standard support for either finding the thread id of the parent of the current thread, or for something like the thread-local references I have proposed? Thanks, Frederik