super fast

gistfile1.hs

getPrime :: Integer -> Integer
getPrime n = head $! filter (\x -> n `mod` x == 0) ([2..(sqrtIntn (realToFrac n))] ++ [n]) 
         where  sqrtIntn n = ceiling (sqrt n) 
       
primeList :: Integer -> [Integer]
primeList n | prime == n = [n]
            | otherwise =  prime : primeList (n `div` prime)
                where prime = getPrime n  
                
problem3 n = last $ primeList n