Haskell Example for CS2104

HaskellEgs.hs

### Eliminate consecutive duplicates of list elements. ###

compress []     = []
compress (x:xs) = [x] ++ (compress $ dropWhile (== x) xs

### Flatten a nested list structure. ###

flatten :: NestedList a -> [a]
flatten (Elem a   )   = [a]
flatten (List (x:xs)) = flatten x ++ flatten (List xs)
flatten (List [])     = []


### Pack consecutive duplicates of list elements into sublists. If a list contains repeated
elements they should be placed in separate sublists. 
###

Example:

(pack '(a a a a b c c a a d e e e e))
((A A A A) (B) (C C) (A A) (D) (E E E E))

pack :: (Eq a) => [a] -> [[a]]
pack = foldr func []
    where func x []     = [[x]]
          func x (y:xs) =
              if x == (head y) then ((x:y):xs) else ([x]:y:xs)


### Duplicate the elements of a list. ###

dupli [] = []
dupli (x:xs) = x:x:dupli xs

### Drop every N'th element from a list. ###

dropEvery :: [a] -> Int -> [a]
dropEvery [] _ = []
dropEvery list count = (take (count-1) list) ++ dropEvery (drop count list) count      

### Split a list into two parts; the length of the first part is given. ###

Example:
split "abcdefghik" 3
("abc", "defghik")

Solution using take and drop:
split xs n = (take n xs, drop n xs)

### Prime Numbers using the Sieve ###

primesR a b = takeWhile (<= b) $ dropWhile (< a) $ sieve [2..]
  where sieve (n:ns) = n:sieve [ m | m <- ns, m `mod` n /= 0 ]

### concatMap ###

concatMap               :: (a -> [b]) -> [a] -> [b]
concatMap f             =  foldr ((++) . f) []