A version that should confuse people not familiar with Haskell even more: primes = 2 : filter isPrime [3..] isPrime x = all (\p -> x `mod` p /= 0) $ ta

A version that should confuse people not familiar with Haskell even more: primes = 2 : filter isPrime [3..] isPrime x = all (\p -> x `mod` p /= 0) $ takeWhile (\p -> p * p <= x) primes primes is a list of prime numbers. It is defined as number 2 and numbers 3,4,5... for which isPrime is True. isPrime is a function that checks that x is a prime number by taking numbers whose squares don't exceed x from... primes list... and then making sure they all divide x with a reminder. It only works because isPrime never touches an unevaluated element of primes list. Otherwise, the program would loop.