10.1

calc.hs

import Control.Monad (mplus)

data Token = TInt Int
           | Operator (Int -> Int -> Int)

sample :: String
sample = "10 4 3 + 2 * -"

calc :: String -> Int
calc = calc' [] . tokenize

calc' :: [Int] -> [Token] -> Int
calc' [n] [] = n
calc' stack (TInt n : ts) = calc' (n : stack) ts
calc' (a:b:ss) (Operator o : ts) = calc' (o b a : ss) ts
calc' _ _ = error "calc error"

tokenize :: String -> [Token]
tokenize = map token . words

operatorTable :: [(String, Int->Int->Int)]
operatorTable = [("+", (+)), ("-", (-)), ("*", (*))]

token :: String -> Token
token s = case token' s of
  Just x -> x
  Nothing -> error "parse error"

token' :: String -> Maybe Token
token' s = (Operator `fmap` lookup s operatorTable)
   `mplus` (TInt `fmap` readMaybe s)
  
readMaybe :: (Read a) => String -> Maybe a
readMaybe s = case reads s of
  [(x, "")] -> Just x
  _ -> Nothing