[Haskell-beginners] Stack overflow folding a map

[Mateusz Kowalczyk]

[1] might help you.

[1] - http://www.haskell.org/haskellwiki/Foldr_Foldl_Foldl%27

Mateusz K.

On 16/02/13 09:13, tcollin371 wrote:
> I've been playing around with Haskell on and off for a while, and am getting a "Stack space overflow" that I can't seem to correct.  I have a map containing about 2 million items.  Actually, it's a map keyed off of an Int where the element is another map keyed off of a string that has three Ints as its data.  The number of top level maps is pretty small (< 50), and the maps held by them hold a lot of entries.
> 
> I want to walk through all of the entries and gather some statistics, but I keep getting the stack overflow.  I worked down my test to an inner and outer fold that just counts the entries, and I still get the stack overflow.  I added 'seq' calls to make sure that the count isn't building up a bunch of undone operations, and I'm using foldlWithKey, and have also tried foldlWithKey' with the same result.
> 
> I get the count in my main function and immediately print it out.  Here is the relevant code:
> 
> outputIfNotFilteredCount1 :: Int -> KeyMap -> PuzzleMapRecord -> Int
> outputIfNotFilteredCount1 inCount key entry
>   = seq inCount (inCount + 1)
> 
> outputDatabaseCount1 :: Int -> Int -> InnerDB -> Int
> outputDatabaseCount1 inCount smndCount innerDB 
>   = seq inCount (inCount + outCount)
>   where
>     outCount
>       = Map.foldlWithKey' outputIfNotFilteredCount1 0 innerDB
> 
> main = do
> ...
>     let
>       count   = Map.foldlWithKey' outputDatabaseCount1 0 resultDB
>     putStrLn $ "Database entries:" ++ (show count)
> 
> 
> Any thoughts on why this is running out of stack space?
> 
> -Truman
> 
> 
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