prime generation by trial division testing

gistfile1.hs

checkd :: [Int] -> Int -> Int 
checkd [] n = n 
checkd (prime:primes) n = 
    if (elem n primes || n `mod` prime == 0) then
        prime
    else
        checkd primes n

preplist :: [Int] -> Int -> [Int]
preplist curlist n = filter (\ x -> x < (n `div` 2 + 1)) curlist

gen :: Int -> Int -> [Int] -> [Int]
gen a b xs =
    if a <= b then gen (a + 2) b newxs else xs
    where
        prime = checkd (preplist xs a) a
        newxs = if prime == a then
                    xs ++ [prime]
                else
                    xs