[Haskell-cafe] stack overflow pain

[Tim Docker]

On 21/09/11 02:39, Heinrich Apfelmus wrote:
> Tim Docker wrote:
>> I'm getting a stack overflow exception in code like this:
>>
>>         -- applyAction :: A -> IO [B]
>>
>>         ....
>>         vs <- fmap concat $ mapM applyAction sas
>>         return vs
>>
>> I don't get it if I change the code to this:
>>
>>         -- applyAction :: A -> IO [B]
>>
>>         ....
>>         mapM_ applyAction sas
>>         return []
>>
>> But of course, I need the results from the actions. I know that
>> the returned list contains approximately 1 million items.
>>
>> Any suggestions on how I should rewrite the first code snippet
>> to not blow the stack?
>
> Of course, a list of 1 million items is going to take a lot of memory, 
> unless you generate it lazily. Unfortunately  mapM  cannot generate 
> its result lazily because it has to execute all IO actions before 
> returning the list of results.

I'm OK with it taking a lot of memory. I should have enough. It's the 
stack overflow exception I'm struggling with.

> I'm not entirely sure whether the stack overflow happens in this part 
> of your code, though. What happens if you change it to
>
>     map_ applyAction sas
>     return [1..1000000]
>
> ? If this still throws a stack overflow, then problem is in the part 
> of the code that consumes said list.

I believe the error is happening in the concat because there are 
subsequent IO actions that fail to execute. ie the code is equivalent to:

         vs <- fmap concat $ mapM applyAction sas
         someOtherAction
         consume vs

and someOtherAction seems not to be run. However, to be sure, I'll 
confirm with code akin to what you suggest above.

Tim