[Haskell-cafe] How to increment an Int in Haskell (stack overflow issue)

[Daniel Fischer]

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