Il Fri, Sep 01, 2006 at 07:22:02AM +0200, Tomasz Zielonka ebbe a scrivere:
On Fri, Sep 01, 2006 at 01:13:14AM +0200, Benjamin Franksen wrote:
So getting the value out of the monad is not a pure function (extract :: Monad m => m a -> a). I think I stated that, already, in my previous post.
The only generic way of "extracting" values from a monadic value is the bind operator. The lack of extract function is a feature :-) But now I know that you are not really claiming such a function exists.
I do not understand this discussion, but I'd like to. Can you please tell me what you are talking about in terms of this example? Thanks, Andrea module Test where newtype M a = TypeConstructor {unpack::(a, String)} deriving (Show) instance Monad M where return a = (TypeConstructor (a,"")) (>>=) m f = TypeConstructor (a1,b++b1) where (a,b) = unpack m (a1,b1) = unpack (f a) putB b = TypeConstructor ((),b) putA a = (TypeConstructor (a,"")) getA (TypeConstructor (a,b)) = a getB (TypeConstructor (a,b)) = b transformM :: Int -> M Int transformM a = do putA 3 putB "ciao" putA 6 putB " cosa?" return 4 {- *Test> let a = transformM 1 *Test> a TypeConstructor {unpack = (4,"ciao cosa?")} *Test> getA a 4 *Test> getB a "ciao cosa?" *Test> -}