Il Tue, Aug 29, 2006 at 10:02:38AM +0100, Brian Hulley ebbe a scrivere:
Yes I agree the StateT/monad transformer approach is probably best in the long run, since by using the standard monad transformers, you will get code that will scale better to handle more complexities later, and has the advantage of being already tested so you can be sure the resulting monads will obey all the monad laws. Also, there are a lot of tutorials about how to use them to solve different problems. ... Hi! It's been quite troublesome since there are no examples (at least I did not find any), but I implemented (copied...;-) StateT.
Below the code. What do you think (apart from names or lack of class instance: I need this code to understand what's going on and to write about it in my tutorial)? Is it quite canonical? Anything really bad?
Happy monadic explorations! :-)
Great fun indeed! Thanks for the kind help from you, guys! Andrea Here's the bit. At the end the output. module Monadi where data Term = Con Int | Add Term Term deriving (Show) eval :: Term -> Int eval (Con a) = a eval (Add a b) = eval a + eval b answer, noanswer :: Term answer = (Add (Add (Con 30) (Con 12)) (Add (Con 20) (Con 30))) noanswer = (Add (Add (Con 20) (Con 12)) (Con 11)) formatLine t a = "eval (" ++ show t ++ ") <= " ++ show a ++ " - " type Exception = String type O = String data M2 a = Ex Exception | Done {unpack :: (a,O) } deriving (Show) newtype StateT s m a = S {runStateT :: s -> m (a,s) } --S (s -> m (a,s)) instance Monad m => Monad (StateT s m) where return a = S (\s -> return (a,s)) S m1 >>= k = S (\s -> do ~(a,s1) <- m1 s let S m2 = k a m2 s1) instance Monad M2 where return a = Done (a, "") m >>= f = case m of Ex e -> Ex e Done (a, x) -> case (f a) of Ex e1 -> Ex e1 Done (b, y) -> Done (b, x ++ y) lift m = S (\s -> do x <- m return (x,s)) raise_IOE :: O -> StateT s M2 a raise_IOE e = lift (Ex e) print_IOE :: O -> StateT Int M2 () print_IOE x = lift (Done ((), x)) incState :: StateT Int M2 (M2 ()) incState = S (\s -> return (Done ((), ""), s + 1)) eval_IOE :: Term -> StateT Int M2 Int eval_IOE (Con a) = do incState print_IOE (formatLine (Con a) a) return a eval_IOE (Add t u) = do a <- eval_IOE t b <- eval_IOE u incState print_IOE (formatLine (Add t u) (a + b)) if (a+b) == 42 then raise_IOE "The Ultimate Answer Has Been Computed!! Now I'm tired!" else return (a + b) -- *Monadi> runStateT (eval_IOE answer) 0 -- Ex "The Ultimate Answer Has Been Computed!! Now I'm tired!" -- *Monadi> runStateT (eval_IOE noanswer) 0 -- Done {unpack = ((43,5),"eval (Con 20) <= 20 - eval (Con 12) <= 12 - eval (Add (Con 20) (Con 12)) <= 32 - eval (Con 11) <= 11 - eval (Add (Add (Con 20) (Con 12)) (Con 11)) <= 43 - ")} -- *Monadi>