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                        minChunksSize=1000.0 -- 100.0
 
facbig :: Integer -> Integer
facbig n = 
        let 
                divisionProportion = 0.9 -- 2.0/3.0
                amountOfChunk = truncate $ logBase divisionProportion $ minChunksSize/fromIntegral n
                proportions = map (divisionProportion^) [amountOfChunk, amountOfChunk-1 .. 1]
                centralChunkResults = map product [
                        let
                                begin = proportionToItemNumber 1 beginRangeProportion
                                end = proportionToItemNumber 0 endRangeProportion
                        in
                        [begin..end] | (endRangeProportion, beginRangeProportion) <- zip proportions $ tail proportions ]
                beginChunkResult = product [1 .. proportionToItemNumber 0 $ last proportions]
                endChunkResult = product [proportionToItemNumber 1 $ head proportions .. n]
        in product $ beginChunkResult:endChunkResult:centralChunkResults
        where
                proportionToItemNumber shift proportion =
                        shift + truncate ((1.0-proportion)*fromIntegral n)
 
fac n = if n <= (truncate $ 3.0*minChunksSize)
        then product [1..n]
        else facbig n
 
main = print $ length $ show $ fac 100000
                                 
        
А вообще читаем алгоритм вычисления факториала тут:
http://gmplib.org/manual/Factorial-Algorithm.html#Factorial-Algorithm
http://www.willamette.edu/~fruehr/haskell/evolution.html