Hi, I have a program, abstracted from a larger application that I am writing for a customer, that persistently overflows its stack. The program is a simulation of the communication protocol of a sensor tag. The code is below. The program mimics a hardware state machine. In the example below, the internal state is just a counter and a another register that holds what is called the tag's "state": Syncing, Listening or Sleeping. The simulation just advances the tags internal state until the counter reaches zero. (In the real application, there are external inputs that can change the state, but that's not needed to see the problem.) The simulation crashes, running out of stack space after only about 400000 cycles on my machine (OS X 10.4.7 ppc). Both hugs and ghci show it: hugs -98 Test2.hs Hugs mode: Restart with command line option +98 for Haskell 98 mode Type :? for help Main> main ERROR - Garbage collection fails to reclaim sufficient space Main> and ghci: Prelude> :load "/Users/gwright/src/haskell/simulator/test2.hs" Compiling Main ( /Users/gwright/src/haskell/simulator/ test2.hs, interpreted ) Ok, modules loaded: Main. *Main> main FrozenTag {ft_tagID = 1, ft_state = *** Exception: stack overflow *Main> Searches through old mailing lists warn me that it can be hard to tell if evaluation is truly tail recursive, and I saw a discussion of this in the context of "monadic loops", but I never saw a solution. Perhaps in my sleep deprived condition I am missing the obvious, but any help would be appreciated. Best, Greg -- -- Test the state transformer calculation. -- -- 21 August 2006 -- module Main where import Control.Monad.ST import Control.Monad.Writer import Data.STRef import Maybe data TagState = Syncing | Listening | Sleeping deriving (Eq, Show) -- A structure with internal state: -- data Tag s = Tag { tagID :: Int, state :: STRef s TagState, count :: STRef s Integer } data FrozenTag = FrozenTag { ft_tagID :: Int, ft_state :: TagState, ft_count :: Integer } deriving Show -- Repeat a computation until it returns Nothing: -- until_ :: Monad m => m (Maybe a) -> m () until_ action = do result <- action if isNothing result then return () else until_ action -- Here is a toy stateful computation: -- runTag :: ST s (FrozenTag) runTag = do tag <- initialize until_ (step tag) freezeTag tag initialize :: ST s (Tag s) initialize = do init_count <- newSTRef 1000000 init_state <- newSTRef Syncing return (Tag { tagID = 1, state = init_state, count = init_count }) step :: Tag s -> ST s (Maybe Integer) step t = do c <- readSTRef (count t) s <- readSTRef (state t) writeSTRef (count t) (c - 1) writeSTRef (state t) (nextState s) if (c <= 0) then return Nothing else return (Just c) nextState :: TagState -> TagState nextState s = case s of Syncing -> Listening Listening -> Sleeping Sleeping -> Syncing freezeTag :: Tag s -> ST s (FrozenTag) freezeTag t = do frozen_count <- readSTRef (count t) frozen_state <- readSTRef (state t) return (FrozenTag { ft_tagID = tagID t, ft_count = frozen_count, ft_state = frozen_state }) main :: IO () main = do putStrLn (show (runST runTag))