-- Here is an example of using an "exception monad"
-- The idea is to write the attribute system for recognizing based numbers
--  ... this uses an example in Louden 

data BN = Bn NM BC
     deriving Show
data BC = Octal | Decimal
     deriving Show
data NM = Snm DG NM
        | Znm DG
     deriving Show
data DG = D0|D1|D2|D3|D4|D5|D6|D7|D8|D9
     deriving Show


data ANS a = Ans a | Err String
     deriving Show

instance Monad ANS where
     return x = Ans x
     Err st >>= q = Err st
     Ans x >>= q = q x

base_num:: BN -> ANS Int
base_num (Bn n b)
     = do d <- base_ch b
          n' <- num n d
          return n'

base_ch:: BC -> ANS Int
base_ch(Octal) = do return 8
base_ch(Decimal) = do return 10

num:: NM -> Int -> ANS Int
num (Snm d n) b 
      = do d' <- digit d b
           n' <- num n b
           return (b*n' + d')
num (Znm d) b 
      = do d' <- digit d b
           return d'

digit:: DG -> Int -> ANS Int
digit D0 b = do return 0
digit D1 b = do return 1
digit D2 b = do return 2
digit D3 b = do return 3
digit D4 b = do return 4
digit D5 b = do return 5
digit D6 b = do return 6
digit D7 b = do return 7
digit D8 b = if 8 < b then do return 8 else Err "Digit is not base 8!"
digit D9 b = if 8 < b then do return 9 else Err "Digit is not base 8!"


base_num (Bn (Snm D7 (Snm D9 (Znm D3))) Octal)