# [Haskell] stack overflow - nonobvious thunks?

Fri Jul 29 16:38:46 EDT 2005

```Thanks! I was confused at first because ghci still runs into memory
problems, though no longer stack overflows. ghc runs it beautifully,
though.

-----Original Message-----
From: Amanda Clare [mailto:afc at aber.ac.uk]
Sent: Thursday, July 28, 2005 3:20 AM
To: Dean Herington
Subject: Re: [Haskell] stack overflow - nonobvious thunks?

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

```