[Haskell-cafe] Strange space leak

Grzegorz Chrupala grzegorz.chrupala at computing.dcu.ie
Mon Jul 14 04:43:06 EDT 2008

Hi all,
I just noticed that a tiny change to the program which I posted recently in
the "More idiomatic use of strictness" thread causes a space leak.
The code is:
{-# LANGUAGE BangPatterns, PatternGuards #-}
import Data.List (foldl')
import Data.Char
split delim s
	| [] <- rest = [token]
	| otherwise = token : split delim (tail rest)
   where (token,rest) = span (/=delim) s

main = do
  putStrLn =<< fmap (show . stats ["the","a","and"] . split "<DOC>" . words)

stats ws docs =  foldl' f ((map (const 0) ws),0) docs
    where f (dfs,n) d = let dfs' = zipWith (\w df -> (df + fromEnum (w
`elem` d))) ws dfs
                        in  sum dfs' `seq` (dfs',n+1)

If I change this line:
  putStrLn =<< fmap (show . stats ["the","a","and"] . split "<DOC>" . words)
to this:
  putStrLn =<< fmap (show . stats ["the","a","and"] . split "<DOC>" . words
. map toLower) getContents

suddenly the programs starts using tons of memory instead of running in
small constant space.
What's going on?


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