Parsec и постфиксная нотация

import Text.Parsec
import Text.Parsec.String (Parser)
 
data S = Term Op S S | Atom Char deriving Show
type Op = Char
 
sParser = term <|> atom
term = Term <$> char '+' <*> sParser <*> sParser 
atom = Atom <$> char 'a'
 
main :: IO ()
main =  parse sParser "" "+a++aaa"

import Text.Parsec
import Text.Parsec.String (Parser)
 
data S = Term S S Op | Atom Char deriving Show
type Op = Char
 
fromRest a []        = a
fromRest a ((s, op):rs) = fromRest (Term a s op) rs
 
sParser = do { a <- atom; rs <- many rest; return $ fromRest a rs }
rest = (,) <$> sParser <*> char '+'   
atom = Atom <$> char 'a'
 
main :: IO ()
main = print $ parse (sParser <* eof) "" "aa+aa++a+"

op :: Parsec String [S] ()
op = do
  o <- oneOf "+-"
  x:y:st <- getState
  putState $ Term o y x : st
 
atom :: Parsec String [S] ()
atom = do
  a <- oneOf ['a'..'z']
  modifyState (Atom a :)
 
result :: Parsec String [S] S
result = do
  [x] <- getState
  return x
 
parser :: Parsec String [S] S
parser = skipMany (op <|> atom) >> result
 
main :: IO ()
main = print $ runP (parser <* eof) [] "" "ab+cd++e+"

fromRest a []        = a
fromRest a ((s, op):rs) = fromRest (Term a s op) rs
 
sParser = do { a <- atom; rs <- many rest; return $ fromRest a rs }

sParser = foldl (uncurry . Term . id) <$> atom <*> many rest