Re: [Haskell-cafe] stack overflow when using ST monad

The write*Ref functions, like many "write into data structure" have the common problem of being much lazier than you want. The nextState calls form a lazy thunk. In fact it tries form 10^6 nested thunks to call nextState. So you have to use something like seq to reduce the laziness:

step :: Tag s -> ST s (Maybe Integer) step t = do c <- readSTRef (count t) s <- readSTRef (state t) writeSTRef (count t) (c - 1) let state'=nextState s state' `seq` writeSTRef (state t) state' if (c <= 0) then return Nothing else return (Just c)