(*) Run-length encoding of a list.
Use the result of problem P09 to implement the so-called run-length encoding data compression method. Consecutive duplicates of elements are encoded as lists (N E) where N is the number of duplicates of the element E.
encode xs = map (\x -> (length x,head x)) (group xs)
which can also be expressed as a list comprehension:
[(length x, head x) | x <- group xs]
encode :: Eq a => [a] -> [(Int, a)] encode = map (\x -> (length x, head x)) . group
Or (ab)using the "&&&" arrow operator for tuples:
encode :: Eq a => [a] -> [(Int, a)] encode xs = map (length &&& head) $ group xs
Or with the help of foldr:
encode xs = (enc . pack) xs where enc = foldr (\x acc -> (length x, head x) : acc)