Re: [Haskell-cafe] More Flexible Monad Transformer OR Bad Coding Style

I've never used this myself, but the package mtlx seems to offer one possible solution to this problem by tagging the monad transformers with index types: http://hackage.haskell.org/package/mtlx Cheers, Greg On 08/09/10 12:39, Gábor Lehel wrote:

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 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

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