PlusPow.hs

import Prelude hiding (exp)
import Data.Char
import Text.ParserCombinators.ReadP

data Exp = Plus Exp Exp
         | Pow Exp Exp
         | Num Int
         deriving (Show)

exp :: ReadP Exp
exp = do
  e <- prim
  e' <- exp1 e
  eof
  return e'

exp1 :: Exp -> ReadP Exp
exp1 e1 = do string "+"
             e2 <- exp2
             exp1 (Plus e1 e2)
          +++ exp2' e1
          +++ return e1

exp2 :: ReadP Exp
exp2 = do
  e <- prim
  exp2' e

exp2' :: Exp -> ReadP Exp
exp2' e1 = do string "^"
              e2 <- exp2
              return (Pow e1 e2)
          +++ return e1

prim :: ReadP Exp
prim = do
  n <- num
  return (Num n)

num :: ReadP Int
num = do
  s <- many1 (satisfy isDigit)
  return (read s)