Data commit

Task/Catalan-numbers/Haskell/catalan-numbers.hs

@@ -0,0 +1,24 @@
+-- Three infinite lists, corresponding to the three
+-- definitions in the problem statement.
+
+cats1 :: [Integer]
+cats1 =
+  (div . product . (enumFromTo . (2 +) <*> (2 *)))
+    <*> (product . enumFromTo 1) <$> [0 ..]
+
+cats2 :: [Integer]
+cats2 =
+  1 :
+  fmap
+    (\n -> sum (zipWith (*) (reverse (take n cats2)) cats2))
+    [1 ..]
+
+cats3 :: [Integer]
+cats3 =
+  scanl
+    (\c n -> c * 2 * (2 * n - 1) `div` succ n)
+    1
+    [1 ..]
+
+main :: IO ()
+main = mapM_ (print . take 15) [cats1, cats2, cats3]