# 99 questions/Solutions/10

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## Revision as of 20:37, 20 July 2010

(*) 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]

Or writing it Pointfree (Note that the type signature is essential here to avoid hitting the Monomorphism Restriction):

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