haskell，计算列表中有多少个质数

Main>  f 2 4
1
Main> f 2 10
3

f :: Int -> Int -> Int
f x y 
  | x < y = length [ n | n <- [x..y], y \'mod\' n == 0] 
  | otherwise = 0

x

y

[x+1 .. y-1]

-- note: no need for the otherwise, since [x..y] == [] if x>y
nPrimes a b  =  length $ filter isPrime [a+1 .. b-1]

n <- [x..y]

x

y

y `mod` n == 0

length (...)

n

f 2 4

[2, 4]

f 2 10

primes = -- A list with all prime numbers
candidates = [a+1 .. b-1]
myprimes = intersectSortedLists candidates primes
nPrimes = length $ myprimes