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?) 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) ... }
to increment them I initially just simply applied either of
incFst, incSnd :: (Int,Int) -> (Int,Int) incFst (x,y) = (x + 1,y) incSnd (x,y) = (x, y + 1)
to the current pair. I thought this was safe since primitive arithmetic was strict, but when I go to print the counter out (evaluate it), it blows apart. I first decided this was because the addition was somehow not strict and I was getting: (0 + 1 + 1 ...millions of times..., ....) and the evaluation of that first coordinate blew things apart. So I tried was adding a strictness annotation to the arguments.
incFst (!x,y) = (x + 1,y) incSnd (x,!y) = (x, y + 1)
No dice (I also tried using seq manually all over the place to no avail). Just for fun, I tried both of
incFst _ = (0,0) incSnd _ = (0,0)
and
incFst (x,y) = (x,y) incSnd (x,y) = (x,y)
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)}
and the monad transformer I am using is
import Control.Monad.State.Strict