[Haskell-cafe] More idiomatic use of strictness

[Jonathan Cast]

On Thu, 2008-07-10 at 03:16 -0700, Grzegorz Chrupala wrote:
> Hi all,
> 
> Is there a less ugly way of avoiding laziness in the code pasted below then
> the use of seq in the last line?
> The program is supposed to split a large input file into chunks and check in
> how many of those chunks each of a list of words appear, as well as the
> total number of chunks. Without the seq it consumes huge amounts of memory.

Strategies!  Try

((,) $| rnf) dfs' (n + 1)

Or

(id $| seqPair rnf r0) (dfs', n + 1)

But I don't know if that falls within the intended meaning of `less
ugly'.

jcc


> {-# LANGUAGE BangPatterns, PatternGuards #-}
> import Data.List (foldl')
> 
> 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)
> getContents
> 
> 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)