Yes, the function fails for [10..30]. But do you ever read the explanations?
primes [10..30] = [10,11,12,13,14,15,16,17,18,19,21,23,25,27,29] -- not bad actually, works as advertised.FAIL
primes [10..30] = [10,11,12,13,14,15,16,17,18,19,21,23,25,27,29] -- not bad actually, works as advertised.