Am Mittwoch 25 März 2009 20:00:40 schrieb Tim Bauer:
I have a program that is currently blowing out the stack, Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize' to increase it. I am pretty sure I get to the end of the computation that increments various statistic counters (lazily?)
Far too lazily.
and only when I go to print them out at the end, do things fail.
I have state monad transformer (StateT) that is keeping the counters among other things. The counters are stored in a pair within a larger data structure.
data AState s a = AS { ... asStats :: (Int,Int) ... }
The problem goes away. But maybe an optimization is covering up the crime. So I tried
incFst (x,y) = (y,x) incSnd (x,y) = (y,x)
And indeed this again crashes. Any hints as to what is going on?
If it is relevant, here is my code to access the counter within the state monad
countFst :: StateT (AState s a) IO () countFst = modify $ \as -> as{asStats = incFst (asStats as)}
This is where the strictness you added to incFst is hidden again. countFst writes the thunk (\as -> as{asStats = incFst (asStats as)}) to the state, incFst isn't even called yet. To force the evaluation, a) make the stats two strict (!Int) fields b) evaluate the state, like in countFst = do as <- get let a = asStats1 as as1 = as{asStats1 = a+1} put $! as1
and the monad transformer I am using is
import Control.Monad.State.Strict