# [Haskell] stack overflow - nonobvious thunks?

Amanda Clare afc at aber.ac.uk
Thu Jul 28 06:20:15 EDT 2005

```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

```