Re: [Haskell-cafe] state and exception or types again...

28 Aug 2006

      Il Mon, Aug 28, 2006 at 08:23:15PM +0200, Andrea Rossato ebbe a scrivere:

The previous code was not complete, and so testable.
at the end there is the output.
there it is:

module Monads where
data Term = Con Int
          | Add Term Term
            deriving (Show)

type State = Int
type Output = String

formatLine :: Term -> Int -> Output
formatLine t a = "eval (" ++ show t ++ ") <= " ++ show a ++ " - "                                                       

data Eval_SOI a = Raise { unPackMSOIandRun :: State -> (a, State, Output) }
                | SOIE { unPackMSOIandRun :: State -> (a, State, Output) }

instance Monad Eval_SOI where
    return a = SOIE (\s -> (a, s, ""))
    m >>= f =  SOIE (\x ->
                       let (a, y, s1) = unPackMSOIandRun m x in
                       case f a of
                         SOIE nextRun -> let (b, z, s2) = nextRun y in  
                                         (b, z, s1 ++ s2)
                         Raise e1 -> e1 y  --only this happens

                      )
--     (>>=) m f =  case m of
--                    Raise e -> error "ciao" -- why this is not going to happen?
--                    SOIE a -> SOIE (\x ->
--                                    let (a, y, s1) = unPackMSOIandRun m x in
--                                    let (b, z, s2) = unPackMSOIandRun (f a) y in 
--                                    (b, z, s1 ++ s2))	   

incSOIstate :: Eval_SOI ()
incSOIstate = SOIE (\s -> ((), s + 1, ""))

print_SOI :: Output -> Eval_SOI ()
print_SOI x = SOIE (\s -> ((),s, x))

raise x e = Raise (\s -> (x,s,e))

eval_SOI :: Term -> Eval_SOI Int
eval_SOI (Con a) = do incSOIstate
                      print_SOI (formatLine (Con a) a)
                      return a
eval_SOI (Add t u) = do a <- eval_SOI t
                        b <- eval_SOI u
                        incSOIstate
                        print_SOI (formatLine (Add t u) (a + b))
                        if (a + b)  ==  42 
                          then raise (a+b) " = The Ultimate Answer!!"
                          else return (a + b)

runEval exp =  case eval_SOI exp of
                 Raise a -> a 0
                 SOIE p -> let (result, state, output) = p 0 in
                             (result,state,output) --"Result = " ++ show result ++ " Recursions = " ++ show state ++ " Output = " ++ output

--runEval (Add (Con 10) (Add (Con 28) (Add (Con 40) (Con 2))))

will produce 
(80,7,"eval (Con 10) <= 10 - eval (Con 28) <= 28 - eval (Con 40) <= 40 - eval (Con 2) <= 2 -  = The Ultimate Answer!!eval (Add (Con 28) (Add (Con 40) (Con 2))) <= 70 - eval (Add (Con 10) (Add (Con 28) (Add (Con 40) (Con 2)))) <= 80 - ")
thats is:
"eval (Con 10) <= 10 -
 eval (Con 28) <= 28 -
 eval (Con 40) <= 40 -
 eval (Con 2) <= 2 -  = The Ultimate Answer!!
 eval (Add (Con 28) (Add (Con 40) (Con 2))) <= 70 -
 eval (Add (Con 10) (Add (Con 28) (Add (Con 40) (Con 2)))) <= 80 -
 "