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