How do I properly use the length function in haskell?-CodePudding

I am currently working on trying to find the number of primes between a range in haskell. The program prints out the range of primes correctly. E.g countPrimesUntil 2 10 will print out [2, 3, 5, 7]. I am looking for the number 4 because that's how many primes is between 2 and 10. How do I incorporate countPrimes correctly?

import Data.List

countPrimesUntil :: Integral a=> a -> a -> [a]
countPrimesUntil a b = takeWhile (<= b) $ dropWhile (< a) $ sieve [2..]
    while sieve (n:ns) = n:sieve [m | m <- ns, m `mod` n /= 0]

countPrimes n = length([x | x <- [2..n], countPrimesUntil x])

CodePudding user response：

countPrimesUntil is misnamed; it doesn't count anything. Rather, it produces a list of primes between a and b, inclusive.

All you need to do is apply length to the result of countPrimesUntil, when given arguments 2 and n.

countPrimes n = length (countPrimesUntil 2 n)
-- countPrimes = length . countPrimesUntil 2

CodePudding user response：

In that case the countPrimes calls countPrimesUntil 2 n, and determines it length, so:

countPrimes n = length (countPrimesUntil 2 n)