I haven't really been following this discussion, but I use Identity-backed monads all the time. Usually some combination of StateT, WriterT, ErrorT and ReaderT. Are there any "base" monads other than IO and Identity?

This technique of newtype'ing stacks to make an abstraction barrier, in conjuction with liftBase, seems useful to me. I can think of at least two (related) projects where I should use it. John On Wed, Apr 21, 2010 at 12:21 AM, Iavor Diatchki wrote:

Hi, A common way to get different "base" monads is to "newtype" existing monads.  Here is an example:

...

newtype CustomMonad a = CM (StateT Int IO a) deriving (Monad)

instance BaseM CustomMonad CustomMonad where inBase = id

... define some custom operations on CustomMonad...

The "newtype" provides an "abstraction barrier", which hides the implementation of the CustomMonad.   If you add more layers on top of "CustomMonad" when you use "inBase" it will only lift up to CustomMonad.   This is useful if, for example, CustomMonad needs to perform some IO actions, but for one reason or another it does not allow for the execution of arbitrary IO computations.

Note that when you are implementing the operations of CustomMonad, you can use "inBase" again, this time to lift all the way down to IO. Basically, by using "newtypes", you can kind of "punctuate" a tall tower of monads, an use "inBase" to jump from one stack to the next.

Hope that this helps, -Iavor

On Tue, Apr 20, 2010 at 2:45 PM, Evan Laforge wrote:

...

I haven't really been following this discussion, but I use Identity-backed monads all the time.  Usually some combination of StateT, WriterT, ErrorT and ReaderT.

Are there any "base" monads other than IO and Identity? _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

Bas van Dijk wrote:

To be honest I also never used a stack of monad transformers that had another base monad than IO. So I also don't see myself using it.

However I can imagine other people will use it or are already using it (users of MonadLib for example) and I'm interested in their use-cases and experiences.

I've used ST as a base monad a number of times. I rarely (if ever) use IO as part of a transformer stack because it has no coherent semantics and is best kept on its own. ST however gives access to mutable references and is very useful for a number of applications. ...Not that I've found the lack of liftST burdensome... -- Live well, ~wren