haskell: add [p4, p10]

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

haskell/Lib.hs

@@ -1,13 +1,19 @@
-module Lib (fib, primeFactors) where
+module Lib (fib, primes, primeFactors) where
 
 fib :: [Integer]
 fib = 1 : 2 : zipWith (+) fib (tail fib)
 
+isPrime :: Integer -> Bool
+isPrime n = n > 1 && null [[] | k <- [2 .. (floor $ sqrt $ fromIntegral n)], mod n k == 0]
+
+primes :: [Integer]
+primes = 2 : filter isPrime [3, 4 ..]
+
 primeFactors :: Integer -> [Integer]
 primeFactors n = genPrimeFactors n 2
   where
     genPrimeFactors :: Integer -> Integer -> [Integer]
     genPrimeFactors n c
-      | c == n = [n]
+      | c * c > n = [n]
       | mod n c /= 0 = genPrimeFactors n (c + 1)
       | otherwise = c : genPrimeFactors (div n c) c