Way back in this thread, Koen Claessen mentioned the idea of a commutative version of the IO monad for handling things with identity. That doesn't quite do it, but I have a refinement that might. The thing is to focus on IO computations that are: a) central -- their effect commutes with every other IO action b) affine -- their effect is not directly observable, and can be discarded. Thus an element u of (IO a) is affine central if for all v::IO b and w::IO c, do { x <- u; v } = v (affine) do { x <- u; y <-v; w } = do { y <- v; x <- u; w } (central) where '=' is observational equivalence. For example, we would expect newMVar x :: IO (MVar a) to be affine central; similarly newIORef, and newStdGen for random number generators. If we define an ACIO monad for affine central IO expression, then we can safely use these in toplevel declarations: declare x <- u -- x::a, u::ACIO a declare count <- newIORef 0 -- count :: IORef Int declare flag <- newMVar True -- flag :: MVar Bool The semantics of such declarations would be to gather them all to the start of a program, whose meaning is now do { declarations; main } Because all declarations are ACIO: they can appear anywhere in the source; be executed at any point between the start of the program and the use of their value; and if there is no such use, need never be executed at all. In any event, the right-hand side of each declaration will be evaluated, and perform its side-effect, at most once. The declared values (x, count, flag) are pure values and do not have IO types. However, almost all their uses (bumping the counter, reading the flag) will live in the IO monad. Like stdIn and stdOut. Also, just because these declarations are toplevel doesn't mean they are globally visible: they are lexically scoped within standard module namespaces. I conjecture that when used "safely" the {-# NOINLINE #-} unsafePerformIO idiom is in fact being applied to IO expressions that are affine central; which means they could be put in ACIO and used as declarations instead. -- Ian Stark http://www.ed.ac.uk/~stark LFCS, School of Informatics, The University of Edinburgh, Scotland