June 2018 Update

Task/Self-describing-numbers/Haskell/self-describing-numbers-1.hs

@@ -4,13 +4,14 @@ count :: Int -> [Int] -> Int
 count x = length . filter (x ==)
 
 isSelfDescribing :: Integer -> Bool
-isSelfDescribing n =
-    nu == f where
-            nu = map digitToInt (show n)
-            f = map (\a -> count a nu) [0 .. ((length nu)-1)]
+isSelfDescribing n = nu == f
+  where
+    nu = digitToInt <$> show n
+    f = (`count` nu) <$> [0 .. length nu - 1]
 
+main :: IO ()
 main = do
-    let tests = [1210, 2020, 21200, 3211000,
-                 42101000, 521001000, 6210001000]
-    print $ map isSelfDescribing tests
-    print $ filter isSelfDescribing [0 .. 4000000]
+  print $
+    isSelfDescribing <$>
+    [1210, 2020, 21200, 3211000, 42101000, 521001000, 6210001000]
+  print $ filter isSelfDescribing [0 .. 4000000]

Task/Self-describing-numbers/Haskell/self-describing-numbers-2.hs

@@ -7,15 +7,16 @@ count x = length . filter (x ==)
 -- all the combinations of n digits of base n
 -- a base-n number are represented as a list of ints, one per digit
 allBaseNNumsOfLength :: Int -> [[Int]]
-allBaseNNumsOfLength n = replicateM n [0..n-1]
+allBaseNNumsOfLength = replicateM <*> (enumFromTo 0 . subtract 1)
 
 isSelfDescribing :: [Int] -> Bool
-isSelfDescribing num =
-  all (\(i,x) -> x == count i num) $ zip [0..] num
+isSelfDescribing num = all (\(i, x) -> x == count i num) $ zip [0 ..] num
 
 -- translate it back into an integer in base-10
 decimalize :: [Int] -> Int
 decimalize = read . map intToDigit
 
-main = forM_ [1..7] $
-  print . map decimalize . filter isSelfDescribing . allBaseNNumsOfLength
+main :: IO ()
+main =
+  (print . concat) $
+  map decimalize . filter isSelfDescribing . allBaseNNumsOfLength <$> [1 .. 8]