Question

I have been learning Haskell over the last few days, through Learn You A Haskell. I’ve been attempting to complete some Project Euler problems, some of which require primes. However the function I have written to try to generate some (in this case primes below 20000) isn’t outputting correctly. When I run it, GHCi returns ‘[1, ‘ and seemingly doesn’t terminate. The code I am using is:

sieve :: (Integral a) => a -> [a] -> [a]
sieve 20000 list = list
sieve n (x:xs) = sieve (n+1) $ x:(filter (\q -> q `mod` n /= 0) xs)

primesto20000 = sieve 2 [1..20000]

And then I am calling primesto20000. I understand that the function may be inefficient, I am mainly asking for help on syntactic/process errors that I must have made.
Thankyou

Answer 1

You’re filtering out multiples of every number, not just prime numbers. You want to check divisibility by x, not by n. (In fact, I’m not sure you need n in the sieve function at all; just make your primesto20000 function generate the appropriate input list, and pass that.)