5, 6, 7. We rock.

4.hs

@@ -1 +1,7 @@
 -- Find the number of elements of a list.
+
+myLength :: [a] -> Int
+myLength [] = 0
+myLength (x:xs) = 1 + myLength xs
+
+main = print (myLength [1..100])

5.hs

@@ -0,0 +1,8 @@
+-- Reverse a list.
+
+myReverse :: [a] -> [a]
+myReverse [] = []
+myReverse [x] = [x]
+myReverse (x:xs) = (myReverse xs) ++ [x]
+
+main = print (myReverse [1..10])

6.hs

@@ -0,0 +1,7 @@
+-- Find out whether a list is a palindrome. A palindrome can be read forward
+-- or backward; e.g. (x a m a x).
+
+myPalindrome :: Eq a => [a] -> Bool
+myPalindrome xs = xs == reverse xs
+
+main = print (myPalindrome "racecar")

7.hs

@@ -0,0 +1,10 @@
+-- Flatten a nested list structure.
+
+data NestedList a = Elem a | List [NestedList a]
+
+myFlatten :: NestedList a -> [a]
+myFlatten (Elem x) = [x]
+myFlatten (List []) = []
+myFlatten (List (x:xs)) = myFlatten x ++ myFlatten (List xs)
+
+main = print (myFlatten (List [Elem 1, List [Elem 2, List [Elem 3, Elem 4], Elem 5]]))