Haskell 99 Problems

haskellproblems.hs

{- Haskell 99 Problems
 - https://wiki.haskell.org/99_questions/1_to_10
 -}

reverse' :: [a] -> [a]
reverse' [] = []
reverse' (x:xs) = (reverse' xs) ++ [x]


myLast :: [a] -> a
myLast [] = error "no last element"
myLast [x] = x
myLast (x:xs) = myLast xs


myButLast :: [a] -> a
myButLast [] = error "no second-to-last element"
myButLast [x,y] = x
myButLast (x:xs) = myButLast xs

myButLast' = last . init


elementAt :: [a] -> Int -> a
elementAt [] _ = error "No element in empty list"
elementAt (x:xs) 0 = x
elementAt (x:xs) n = elementAt xs (n-1)


myLength :: [a] -> Int
myLength [] = 0
myLength (x:xs) = 1 + myLength xs


isPalindrome :: (Eq a) => [a] -> Bool
isPalindrome [] = True
isPalindrome [x] = True
isPalindrome xs = (a == b) && isPalindrome (tail (init xs))
    where
        a = head xs
        b = last xs


compress :: (Eq a) => [a] -> [a]
compress [] = []
compress [x] = [x]
compress (x:y:zs)
    | x == y    =   compress (y:zs)
    | otherwise =   x : compress (y:zs)


pack :: (Eq a) => [a] -> [[a]]
pack [] = []
pack (x:xs) = group : pack remaining
    where
        group = takeWhile (==x) (x:xs)
        remaining = dropWhile (==x) (x:xs)