[Haskell-cafe] Re: Processing of large files

Alexander Kogan alexander at kogan.nnov.ru
Tue Nov 2 02:48:18 EST 2004


> The list of tuples _does_ need to be strict. Beyond that, as Ketil Malde
> said, you should not use foldl -- instead, foldl' is the best version to
> use when you are recalculating the result every time a new list item is
> processed.

Thanks! I did the following:

merge [] x = [(x,1)]
merge (e@(a,b):xs) x | x == a    = let b' = b+1 in b' `seq` (a,b'):xs
                     | otherwise = e : merge xs x

foldl' f z xs = lgo z xs
        lgo z []     =  z
        lgo z (x:xs) = (lgo $! (f z x)) xs

procFile =
    putStrLn        .
    show            .
    foldl' merge []  .

and it works!
But I wonder why the very useful function foldl' as I define it is not 
included into Prelude? I think, many people work with large lists or 

Alexander Kogan
Institute of Applied Physics
Russian Academy of Sciences

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