Stack space overflow when computing primes

Question

I\'m working my way through Real World Haskell (I\'m in chapter 4) and to practice a bit off-book I\'ve created the following program to calculate the nth prime.

<         


        

Answer 1

As I said in my comment, you shouldn't be building a list by appending a single element list to the end of a really long list (your line primes' = primes ++ [test]). It is better to just define the infinite list, primes, and let lazy evaluation do it's thing. Something like the below code:

primes = [2, 3] ++ loop 5
    where.
        loop test
            | isPrime primes test = test:(loop test')
            | otherwise = test' `seq` (loop test')
            where
                test' = test + 2

Obviously you don't need to parameterize the isPrime function by primes either, but that's just a nit. Also, when you know all the numbers are positive you should use rem instead of mod - this results in a 30% performance increase on my machine (when finding the millionth prime).