Продолжаем играть в гольф.

-- Interface

head (mySeq "1")
-- "1"
take 7 (mySeq "1")
-- ["1","11","21","1211","111221","312211","13112221"]
find ((> 15) . length) (mySeq "1")
-- Just "13211311123113112211"

-- Implementation
mySeq :: String -> [String]
mySeq = iterate $ concatMap (\chs -> show (length chs) ++ take 1 chs) . group