left-recursive expression reduction example

term.hs

import Text.Parsec

data Term = Plus Term Term
          | Minus Term Term
          | Div Term Term
          | Mult Term Term
          | Number Integer
          deriving (Show)

type TermParser a = Parsec String () a

opParser :: TermParser (Term -> Term -> Term)
opParser = choice [ char '+' >> return Plus
                  , char '-' >> return Minus
                  , char '/' >> return Div
                  , char '*' >> return Mult ]

numberParser :: TermParser Term
numberParser = Number . (read :: String -> Integer) <$> many1 digit

termParser :: TermParser Term
termParser = chainl1 numberParser opParser

-- > runParser termParser () "" "231+4321*434"
-- Right (Mult (Plus (Number 231) (Number 4321)) (Number 434))