parser.hs

import Control.Monad.State

data Tree = Tree [Tree] | Node String deriving Show


type Parse a = (a, String)

(<~>) :: Parse a -> (a -> b) -> Parse b
(x,y) <~> f = (f x, y)

(<:>) :: Parse a -> (String -> Parse [a]) -> Parse [a]
(a,b) <:> f = (f b) <~> (a:)

parseTree :: String -> Parse Tree
parseTree xs = parseNodes xs <~> Tree

parseNodes :: String -> Parse [Tree]
parseNodes [] = ([], "")
parseNodes ('(':xs) = parseTree xs <:> parseNodes
parseNodes (')':xs) = ([], xs)
parseNodes xs = parseString xs <~> Node <:> parseNodes

parseString :: String -> Parse String
parseString "" = ("", "")
parseString xxs@(x:xs) | x `elem` "()" = ("", xxs)
                        | otherwise = parseString xs <~> (x:)