On Sat, Aug 25, 2012 at 03:14:02AM -0700, Dennis Raddle wrote:

I'm not exactly a Haskell beginner but haven't grasped much of it yet. I have a feeling the answer to your question has something to do with the fact that the monad transformer library provides instances that allow the use of, for example, "get" and "put" in any transformed monad that has a StateT somewhere in it. Or throwError in anything that has an ErrorT in it. In other words, you don't need to lift everything into the right level.

Well, not really, that's a separate thing to do with type classes like MonadState and MonadError. To answer the OP's question, literally composing monadic types does often give you a type similar to the transformer version. However, the point of composing monads is to give you a new, combined monad. If you just use the type (State s (Writer w a)) this is just a State computation which happens to return something of type Writer w a. Combining such things with >>= does not take the Writer into account at all. On the other hand, StateT s (Writer w) a is a separate type with a Monad instance that combines the effects of State and Writer. -Brent

On Sat, Aug 25, 2012 at 1:15 AM, Song Zhang wrote:

...

When I use a State Monad transformer to combine with a Writer Monad StateT s (Writer w) a. is it different from composition of State Monad and Writer Monad. It is State s (Writer w a) ? StateT is defined as (s -> m (a, s)), so StateT s (Writer w) a can be regarded as (s -> Writer w a) , which is (s -> ((a,w),s) and on the other hand State s (Writer w a) is (s -> ((a,w),s). I suppose the are similar and if so, what is the point we still get Monad transformers? Thanks

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