Parse indented text structure (algorithm thanks to http://stackoverflow.com/a/27217698/465100)

parse-indent.hs

import Control.Applicative ((<$>))

data Cat = Cat String [Cat] deriving Show

indentLevel :: Int
indentLevel = 4 -- leading spaces

level :: String -> Int
level = (`div` indentLevel) . length . takeWhile (== ' ')

parse :: [String] -> [Cat]
parse [] = []
parse (x : xs) = Cat (trim x) (parse children) : parse sisters
    where
        (children, sisters) = span ((level x <) . level) xs
        trim                = dropWhile (== ' ')

parseStart :: String -> Cat
parseStart = head . parse . lines

main :: IO Cat
main = parseStart <$> readFile "example-indent.txt"

{-
    example-indent.txt:
Cat 1
    Cat 1.1
        Cat 1.1.2
        Cat 1.1.2
    Cat 1.2
Cat 2
    Cat 2.1
        Cat 2.1.2
    Cat 2.2
        Cat 2.2.2
-}