Ninety-Nine Haskell Problems: Solutions

99.hs

last' :: [a] -> a
last' [x] = x
last' (_:xs) = last xs

butLast :: [a] -> a
butLast [x,_] = x
butLast (_:xs) = butLast xs

elementAt :: Int -> [a] -> a
elementAt 0 xs = head xs
elementAt n (x:xs) = elementAt (n-1) xs

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

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

isPalindrome :: Eq a => [a] -> Bool
isPalindrome [] = True
isPalindrome [_] = True
isPalindrome xs
    | last xs == head xs = isPalindrome $ init $ tail xs
    | otherwise          = False

-- No nested list structure in Haskell.

compress :: Eq a => [a] -> [a]
compress [] = []
compress [x] = [x]
compress (x:xs)
    | x == head xs = compress xs
    | otherwise    = x:compress xs