June 2018 Update

Task/Pernicious-numbers/Haskell/pernicious-numbers-2.hs

@@ -0,0 +1,26 @@
+import Data.List (unfoldr)
+import Data.Tuple (swap)
+
+isPernicious :: Int -> Bool
+isPernicious = isPrime . popCount
+
+popCount :: Int -> Int
+popCount =
+  sum . unfoldr ((flip if_ Nothing . (0 ==)) <*> (Just . swap . flip quotRem 2))
+
+isPrime :: Int -> Bool
+isPrime =
+  (==) <$> (\n -> [1 .. n] >>= flip ((if_ . (0 ==) . mod n) <*> return) []) <*>
+  ((1 :) . return)
+
+if_ :: Bool -> a -> a -> a
+if_ True x _ = x
+if_ False _ y = y
+
+main :: IO ()
+main =
+  mapM_
+    print
+    [ take 25 $ filter isPernicious [1 ..]
+    , filter isPernicious [888888877 .. 888888888]
+    ]