[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)


 > 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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