Actually, while I haven't even used monad transformers before (just read about them a lot), I was thinking that something like this might be the way to solve the lift . lift . lift . lift . foo problem on the one hand, and by wrapping the 'contents' (e.g. the environment of a reader monad) of every level of the stack in a unique newtype (if the type isn't otherwise unique), the problem of "what if I want to use the same transformer more than once, how do I disambiguate them". (Do I have roughly the right idea?)
On Mon, Aug 9, 2010 at 9:05 PM, aditya siram <aditya.si...@gmail.com> wrote: > Hi all, > I was experimenting with monad transformers and realized that the stacking > order of the monads can remain unknown until it is used. Take for example > the following code: > > import "mtl" Control.Monad.State > import "mtl" Control.Monad.Writer > import "mtl" Control.Monad.Identity > > test :: (MonadWriter [Char] m, Num t, MonadState t m) => m () > test = do > put 1 > tell "hello" > > main = do > x <- return $ runIdentity $ runStateT (runWriterT test) 1 -- test :: > WriterT String (StateT Int Identity) > y <- return $ runIdentity $ runWriterT $ runStateT test 1 -- test :: > StateT Int (WriterT String Identity) > z <- runWriterT $ runStateT test 1 -- test :: > StateT Int (WriterT String IO) (((), Int), String) > print x > print y > print z > > *Main> main > (((),"hello"),1) > (((),1),"hello") > (((),1),"hello") > > Until test is called in 'main' we don't know the order of monads. In fact > even the base monad is not know. All we know is that it uses the State and > Writer monad. In each call to 'test' in main we can determine the stacking > order and the base monad yielding different results. This seems to be a more > flexible way of using monad transformers but I haven't seen this in code > before so is there anything wrong with this style? > > -deech > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > -- Work is punishment for failing to procrastinate effectively. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
