Project Euler Problem 010

import Control.Monad
import Control.Monad.ST
import Data.Array.ST
import Data.Array.Unboxed
import Data.List

--無限リストを使ったエラトステネスの篩a
primes :: [Int]
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

n :: Int
n=2000000 :: Int
ans1 :: Int
ans1 = sum $ takeWhile ( UArray Int Bool
era n = runSTUArray(
  do{ a <- newArray (1,n) True :: ST s (STUArray s Int Bool);
      writeArray a 1 False;
      sequence_[do{ prime <- readArray a i;
                    when prime (sequence_[do{writeArray a k False} | k<-[i+i,i*3..n]]);
      } |i<-[1..n]];
      return a;
      })

-- 003より。O(n^1.2)の篩
primes2 :: [Integer] --このアノテーションはずすとバグるので注意する
primes2 = 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