[Haskell-cafe] stack overflow when using ST monad

[Gregory Wright]

Hi Udo,

On Aug 24, 2006, at 7:22 AM, Udo Stenzel wrote:

> Hi Gregory,
>
> Gregory Wright wrote:
>> step :: Tag s -> ST s (Maybe Integer)
>> step t = do
>>         c <- readSTRef (count t)
>>         s <- readSTRef (state t)
>>         writeSTRef (count t) (c - 1)
>>         writeSTRef (state t) (nextState s)
>>         if (c <= 0) then return Nothing else return (Just c)
>
> just looking at the program, this seems to be the problem: writeSTRef
> does not force the evaluation of the stored value.  So after repeated
> calculation, you end up storing not the current counter and state, but
> something like (nextState (...(nextState (nextState initState))...)).
> The counter is evaluated for the conditional at the end, so it doesn't
> exhibit this problem.  Your computation runs to its end, then that
> deeply nested expression is evaluated and exhausts the control stack.
> Try this instead:
>
>>         writeSTRef (state t) $! nextState s
>
> If TagState is a more complicated data type, you may also need strict
> fields in there.
>
> [This comes up so often, shouldn't there be an FAQ about it  
> somewhere?  It
> could even offer a guideline along the lines of "Whenever you  
> repeatedly
> update some value, chances are that you want to force strict
> evaluation."]
>

I agree this should be a FAQ.  Perhaps I should write it up for the
performance section of the wiki?  Looking back I see my mental error
was that I thought I was doing what you and everyone else correctly
suggested:

	writeSTRef (state t) $! nextState s

but what I actually typed was

	writeSTRef (state t) (nextState $! s)

which of course doesn't help.  Another telling example
of the fact that coffee is not an entirely adequate substitute for
sleep.

Best,
Greg

>
> Udo.
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