{-# OPTIONS_GHC -fno-warn-missing-methods #-}
-- ^ option for exercise 6

-- Exercise 1

fib :: Integer -> Integer
fib 0 = 0
fib 1 = 1
fib n = (fib (n-1)) + (fib (n-2))

fibs1 :: [Integer]
fibs1 = map fib [0..]

-- Exercise 2

fibs2 :: [Integer]
fibs2 = map fst $ iterate (\l -> ( snd l , snd l + fst l )) (0,1)

-- Exercise 3

data Stream a = Cons a (Stream a)

streamToList :: Stream a -> [a]
streamToList (Cons a s) = a : streamToList s

instance Show a => Show (Stream a) where
         show s = show $ take 20 $ streamToList s

-- Exercise 4

streamRepeat :: a -> Stream a
streamRepeat e = Cons e (streamRepeat e)

streamMap :: (a -> b) -> Stream a -> Stream b
streamMap f (Cons a as) = Cons (f a) (streamMap f as)

streamFromSeed :: (a -> a) -> a -> Stream a
streamFromSeed f s = Cons s (streamFromSeed f (f s))

-- Exercise 5

nats :: Stream Integer
nats = streamFromSeed (+1) 0

-- The number of times an integer can be evenly divided by 2.
evenDiv2 :: Integer -> Integer
evenDiv2 n = toInteger $ length $ takeWhile even $ iterate (`quot` 2) n

ruler :: Stream Integer
ruler = streamMap evenDiv2 $ streamFromSeed (+1) 1

-- Exercise 6 (Optional)

x :: Stream Integer
x = Cons 0 $ Cons 1 $ streamRepeat 0

instance Num (Stream Integer) where
         fromInteger n = Cons n $ streamRepeat 0
         negate s      = streamMap negate s
         (+) s1 s2     = streamMap (\p -> fst p + snd p) $ zip s1 s2
                       where zip (Cons a0 sa) (Cons b0 sb) = Cons (a0,b0) (zip sa sb)
         (*) (Cons a0 sa) b@(Cons b0 sb) = Cons (a0*b0) $ (streamMap (*a0) sb + (sa*b))