Untitled

import qualified Text.Parsec as P
import Text.Parsec.String (Parser)

data S = SNum Int | SAtom String | SList [S] deriving Show

main :: IO ()
main = do
  P.parseTest s "123"
  P.parseTest s "abc"
  P.parseTest s "+"
  P.parseTest s "()"
  P.parseTest s "(abc 123)"
  P.parseTest s "(abc (+ 2 3))"

s :: Parser S
s = P.choice [parseNum, parseAtom, parseList]

parseNum :: Parser S
parseNum = do
  ds <- P.many1 P.digit
  return $ SNum $ read ds

parseAtom :: Parser S
parseAtom = do
  x <- P.many1 (P.noneOf "() ")
  return $ SAtom x

parseList :: Parser S
parseList = do
  P.char '('
  xs <- P.many $ do
    x <- s
    P.skipMany P.space
    return x
  P.char ')'
  return $ SList xs