Please explain why this fails for negative numbers. By fails I mean it
starts eating RAM infinitely until either me or OOM kills it. If you
replace `m1` definition with the one commented out, the code will fail
for positive integers as well, which is very frustrating.
import System.Environment
isPrime :: Int -> (Bool, String)
isPrime n =
let ll n = k + 1 : k + 5 : ll (n + 1)
where
k = 6 * n + 1
l = 2 : 3 : 5 : ll 1
q (i:is)
| i * i > n = (True, "it is")
| (n `rem` i) == 0 = (False, "divisible by " ++ show i)
| otherwise = q is
in q l
main =
getArgs >>= \argv ->
let f True = "primal"
f False = "not primal"
m0 = map (\x -> read x :: Int) argv
m1 = m0
--m1 =
-- map
-- (\x ->
-- if x < 0
-- then -x
-- else x)
-- m0
m2 = map isPrime m1
msg (x, (y0, y1)) = x ++ " is " ++ f y0 ++ " because " ++ y1
in mapM_ putStrLn $ map msg (zip argv m2)
