Project Euler Problem 035

import Data.Char
import Data.List
--003より
primes :: [Integer]
primes = 2 : ([3,5..] `minus` join [[p*p, p*p+2*p..] | p <- primes']) 
  where
    primes' = 3 : ([5,7..] `minus` join [[p*p, p*p+2*p..] | p <- primes'])   
    join  ((x:xs):t)    = x : union xs (join (pairs t))
    pairs ((x:xs):ys:t) = (x : union xs ys) : pairs t                      
    union (x:xs) (y:ys) = case (compare x y) of 
      LT -> x : union  xs  (y:ys)
      EQ -> x : union  xs     ys 
      GT -> y : union (x:xs)  ys
    union  xs     []    = xs
    union  []     ys    = ys
    minus (x:xs) (y:ys) = case (compare x y) of 
      LT -> x : minus  xs  (y:ys)
      EQ ->     minus  xs     ys
      GT ->     minus (x:xs)  ys
    minus  xs     _     = xs
factor :: Integer -> [Integer]
factor n = factorimpl n primes where
  factorimpl n pri@(p:xs) =
    if p*p>n then [n]
    else if n`rem` p == 0 then
      p:factorimpl (n `quot` p) pri
         else
           factorimpl n xs
divsum :: Integer -> Integer
divsum n =  (product. map tohi .group . factor) n - n where
    tohi [x] = x+1
    tohi l@(x:xs) = (x^(length l+1)-1) `quot` (x-1)
isprime :: Integer -> Bool
isprime nn = imp nn primes where
  imp n (x:xs) = if x*x>n then True
                 else if rem n x==0 then False
                      else imp n xs      

rotate :: Show a => a -> Int
rotate n = let (x:xs) = map digitToInt $ show n in
  foldl1 (\ac x -> x+ac*10) (xs++[x])
digit :: (Integral a, Num a1) => a -> a1
digit x = if x `quot` 10 /= 0 then 1 + digit (x`quot` 10) else 0  
iscir :: Int -> Bool
iscir x = let n = show x in
  if '0' `elem` n then False
  else case length n of
    0 -> True
    n -> all (isprime.fromIntegral) $ take (n+1) $ iterate rotate x
an :: Integer -> [Integer]
an n = [x | x<-takeWhile (