[Haskell-cafe] How to increment an Int in Haskell (stack overflow
issue)
Tim Bauer
bauertim at eecs.orst.edu
Wed Mar 25 15:00:40 EDT 2009
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
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