Project Euler Problem 070

import Data.List
primes :: [Integer]
primes = 2:f [3] [3,5..] where
  f (x:xs) ys = ps ++ f (xs++ps) [z | z<-qs,z`rem`x/=0] where
    (ps,qs) = span (< x*x) ys
factor :: Integer -> [Integer]
factor nn = factorimpl nn 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
limit :: Integer
limit = 10^(7::Integer)

ans1 :: Integer
ans1 =  snd $ minimum a where
  a = [((fromIntegral pq / fromIntegral tot),pq) |
       let lim = takeWhile(<=(limit`quot`2)) primes, --最小の素数は2
       p<-filter ((==2).(`rem`3)) lim, --euler p,qは3を法として2
       q<-filter (\x -> ((x `rem` 3) == 2)&&((p+x-1)`mod`9==0)) $
          takeWhile (\x -> x