Semiprimes (Euler, 187)

-- I put this as a guest first, it was a mistake...

-- ### Project Euler, problem 187 ### --
listPrimes n = takeWhile (<= n) allPrimes
allPrimes = 2 : filter isPrime [3,5..]
  where
    isPrime n = all (/= 0) $ map (n `mod`) [2..squareRoot n]
    squareRoot = truncate . sqrt . fromIntegral

factors n = f n (reverse $ listPrimes n)
  where
    f _ [] = []
    f 1 _ = []
    f m l@(x:xs) | m `mod` x == 0 = x : f (m `div` x) l
                 | otherwise      = f m xs

solveit n = length . filter ((==2) . length) $ map factors [1..n]

main = do
  putStrLn . show $ solveit (10^8)