Added exercise0 from reasoning about programs lesson

haskell/functional-programming-101-edx/reasoning_about_programs/ex0.hs

@@ -0,0 +1,24 @@
+last :: [a] -> a
+last [x] = x
+last (_ : xs) = last xs
+
+foldr :: (a -> b -> b) -> b -> [a] -> b
+foldr _ v [] = v
+foldr f v (x : xs) = f x (foldr f v xs)
+
+init :: [a] -> [a]
+init [_] = []
+init (x : xs) = x : init xs
+
+drop :: Int -> [a] -> [a]
+drop 0 xs = xs
+drop n [] = []
+drop n (_ : xs) = drop (n - 1) xs
+
+(++) :: [a] -> [a] -> [a]
+[] ++ ys = ys
+(x : xs) ++ ys = x : (xs ++ ys)
+
+foldl :: (a -> b -> a) -> a -> [b] -> a
+foldl _ v [] = v
+foldl f v (x : xs) = foldl f (f v x) xs