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Task/Additive-primes/Haskell/additive-primes.hs

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+import Data.List (unfoldr)
+
+-- infinite list of primes
+primes = 2 : sieve [3,5..]
+  where sieve (x:xs) = x : sieve (filter (\y -> y `mod` x /= 0) xs)
+
+-- primarity test, effective for numbers less then billion
+isPrime n = all (\p -> n `mod` p /= 0) $ takeWhile (< sqrtN) primes
+  where sqrtN = round . sqrt . fromIntegral $ n
+
+-- decimal digits of a number
+digits = unfoldr f
+  where f 0 = Nothing
+        f n = let (q, r) = divMod n 10 in Just (r,q)
+
+-- test for an additive prime
+isAdditivePrime n = isPrime n && (isPrime . sum . digits) n