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Какой грязный хак :)

suminit :: [Int] -> Int -> Int -> (Int,[Int]) 
suminit xs len acc | len `seq` acc `seq` False = undefined -- :)))
                   | len == 0  = (acc,xs) 
                   | otherwise = case xs of []   -> (acc,[]) 
                                            x:xs -> suminit xs (len-1) (acc+x)

Найдено тут.