September Morn Update

Task/Multiplication-tables/Haskell/multiplication-tables-1.hs

@@ -0,0 +1,30 @@
+import Data.Maybe (fromMaybe, maybe)
+import Data.Bool (bool)
+
+table :: [Int] -> [[Maybe Int]]
+table xs =
+  let axis = Just <$> xs
+  in (Nothing : axis) :
+     zipWith
+       (:)
+       axis
+       [ [ bool (Just (x * y)) Nothing (x > y)
+         | y <- xs ]
+       | x <- xs ]
+
+-- TEST ---------------------------------------------------
+main :: IO ()
+main =
+  (putStrLn . unlines) $
+  (showTable . table) <$> [[13 .. 20], [1 .. 12], [95 .. 100]]
+
+-- FORMATTING ---------------------------------------------
+showTable :: [[Maybe Int]] -> String
+showTable xs =
+  let w = 1 + (length . show) (fromMaybe 0 $ (last . last) xs)
+      gap = replicate w ' '
+      rows = (maybe gap (rjust w ' ' . show) =<<) <$> xs
+  in unlines $ head rows : [] : tail rows
+
+rjust :: Int -> Char -> String -> String
+rjust n c = (drop . length) <*> (replicate n c ++)

Task/Multiplication-tables/Haskell/multiplication-tables-2.hs

@@ -0,0 +1,11 @@
+import Data.List (groupBy)
+import Data.Function (on)
+import Control.Monad (join)
+
+main :: IO ()
+main =
+  mapM_ print $
+  fmap (uncurry (*)) <$>
+  groupBy
+    (on (==) fst)
+    (filter (uncurry (>=)) $ join ((<*>) . fmap (,)) [1 .. 12])

Task/Multiplication-tables/Haskell/multiplication-tables.hs

@@ -1,26 +0,0 @@
-import Data.Monoid ((<>))
-import Data.List (intercalate, transpose)
-
-multTable :: Int -> [[String]]
-multTable n =
-  let xs = [1 .. n]
-  in xs >>=
-     \x ->
-        [ show x <> ":" :
-          (xs >>=
-           \y ->
-              [ if y < x
-                  then mempty
-                  else show (x * y)
-              ])
-        ]
-
-table :: String -> [[String]] -> [String]
-table delim rows =
-  let justifyRight c n s = drop (length s) (replicate n c <> s)
-  in intercalate delim <$>
-     transpose
-       ((fmap =<< justifyRight ' ' . maximum . fmap length) <$> transpose rows)
-
-main :: IO ()
-main = (putStrLn . unlines . table "  " . multTable) 12