Chapter7.hs

-- Exercise 1
reexpr :: (a -> a) -> (a -> Bool) -> [a] -> [a]
reexpr f p xs = map f (filter (p) xs)

-- Exercise 2
allo :: (a -> Bool) -> [a] -> Bool
allo p = and . map (p)

anyo :: (a -> Bool) -> [a] -> Bool
anyo p = or . map (p)

takeWhileo :: (a -> Bool) -> [a] -> [a]
takeWhileo p [] = []
takeWhileo p (x:xs) | p x = x : takeWhileo (p) xs
                    | otherwise = []

dropWhileo :: (a -> Bool) -> [a] -> [a]
dropWhileo p [] = []
dropWhileo p (x:xs) | p x = dropWhile (p) xs
 | otherwise = x:xs
-- Exercise 3
mapfr :: (a -> b) -> [a] -> [b]
mapfr f [] = []
mapfr f xs = foldr (\x y -> f x : y) [] xs

filterfr' :: (a -> Bool) -> a -> [a]
filterfr' p x | p x = [x]
              | otherwise = []

filterfr :: (a -> Bool) -> [a] -> [a]
filterfr p = foldr (\x y -> (filterfr' p x) ++ y ) []

-- Exercise 4
dec2int :: [Int] -> Int
dec2int = foldl (\x y -> x*10 + y) 0

-- Exercise 5
curryo :: ((a,b) -> c) -> a -> b -> c
curryo f x y = f (x, y)

uncurryo :: (a -> b -> c) -> (a,b) -> c
uncurryo f = \(x,y) -> f x y

-- Exercise 6
unfold :: (t -> Bool) -> (t -> a) -> (t -> t) -> t -> [a]
unfold p h t x | p x = []
               | otherwise = h x : unfold p h t (t x)

chop8 :: [Int] -> [[Int]]
chop8 = unfold null (take 8) (drop 8)

mapo :: (a -> b) -> [a] -> [b]
mapo f = unfold null (f . head) (drop 1)

iterateo :: (a -> a) -> a -> [a]
iterateo f x = unfold (const True) (f . head ) (id) [x]


-- Exercise 9
altMap :: (a -> b) -> (a -> b) -> [a] -> [b]
altMap f1 f2 [] = []
altMap f1 f2 (x:xs) = f1 x : altMap f2 f1 xs