Re: [Haskell-beginners] Stack overflow folding a map

18 Feb 2013


      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 
Sent: Feb 16, 2013 1:13 AM
To: Hask 
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
_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://www.haskell.org/mailman/listinfo/beginners