# [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