[Haskell] stack overflow - nonobvious thunks?

[Amanda Clare]

Dean's version certainly seems the neatest, but just for interest you 
can also do it with a cps fold instead of foldl' too:

table xs = assocs $! cpsfold f empty xs
     where
     f x m k = case Map.lookup x m of
	       Just v  -> v `seq` (k $ Map.adjust (+1) x m)
	       Nothing -> k $ Map.insert x 1 m


cpsfold f a [] = a
cpsfold f a (x:xs) = f x a (\y -> cpsfold f y xs)


As far as I understand it this just makes sure the "seq" happens before 
the folding continues.

When compiled with ghc, both solutions are very well behaved, and seem 
to take the same small amount of memory whether for 10000000 or 100000000.

Amanda


Dean Herington wrote:
> The following version seems to do the trick (and still remain quite 
> readable).  It worked for 100000000 as well.
> 
> import Data.Map as Map
> import System.Random
> import Data.List (foldl')
> 
> table :: (Ord a) => [a] -> [(a,Int)]
> table xs = Map.assocs $! foldl' f Map.empty xs
>     where f m x = let  m' = Map.insertWith (+) x 1 m
>                        Just v = Map.lookup x m'
>                   in v `seq` m'
> 
> unif :: [Int]
> unif = randomRs (1,10) $ mkStdGen 1
> 
> f :: Int -> [(Int, Int)]
> f n = table $ take n unif
> main = print $ f 10000000
> 
> - Dean