[Haskell-beginners] Stack overflow folding a map

[tcollin371]

It turns out that the problem isn't the counting code in the fold operations, but instead seems to be lazy evaluation that's happening as the map is built.  If I run through the count operation every few thousand inserts, then I don't get the stack overflow operation.

-Truman


-----Original Message-----
>From: tcollin371 <tcollin371 at earthlink.net>
>Sent: Feb 16, 2013 1:13 AM
>To: Hask <beginners at haskell.org>
>Subject: [Haskell-beginners] Stack overflow folding a map
>
>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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