Implement a tribonacci sequence

Exercises/exercise-4/Solution.hs

@@ -92,3 +92,48 @@ sieveC pred xs = realSieve id xs
 -- >>> (\ n -> sieve (\ x -> \ y -> (y `mod` x) > 0) [2..n]) 100
 -- [2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,89,97]
 --
+
+--------------------
+-- Exercise 4
+--------------------
+data Tree a = Tree a [Tree a]
+depth :: Tree a -> Integer
+
+-- depth (Tree _ subtrees) =
+--     1 + (maximum $ 0 : (map depth subtrees))
+depth tree = realDepth [tree] 0
+    where
+        realDepth [] x = x
+        realDepth trees x = realDepth (concatMap (\ (Tree _ subtrees) -> subtrees) trees) (x + 1)
+
+-- >>> depth (Tree 1 [Tree 1 []])
+-- 2
+--
+
+-- Notes
+-- realDepth [(Tree 1 [Tree 1 []])] 0
+-- realDepth (concatMap (\ (Tree _ subtrees) -> subtrees) [Tree 1 [Tree 1 []]]) (0 + 1)
+-- realDepth [Tree 1 []] 1
+-- realDepth (concatMap (\ (Tree _ subtrees) -> subtrees) [Tree 1 []]) ((0 + 1) + 1)
+-- realDepth [] 2
+-- 2
+
+fibonacci :: Integer -> Integer
+fibonacci = fib 0 1
+    where
+        fib acc b 0 = acc
+        fib acc b x = fib b (acc + b) (x - 1)
+
+-- >>> fibonacci 6
+-- 8
+--
+
+tribonacci :: Integer -> Integer
+tribonacci = trib 0 0 1
+    where
+        trib acc b c 0 = acc
+        trib acc b c x = trib b c (acc + b + c) (x - 1)
+
+-- >>> tribonacci 7
+-- 13
+--