Re: [Haskell-cafe] Reader monad wrapping State monad

And swap the arguments. Thanks for going the extra mile. Michael --- On Thu, 2/3/11, Daniel Fischer wrote: From: Daniel Fischer Subject: Re: [Haskell-cafe] Reader monad wrapping State monad To: "michael rice" Cc: haskell-cafe@haskell.org Date: Thursday, February 3, 2011, 4:15 PM On Thursday 03 February 2011 21:40:13, michael rice wrote:

Hi Daniel,

Ok, but what I was looking for was ReaderT on top, State on the bottom.

No problem, just change the definition of the Heron type synonym and swap the applcations of runReader[T] and evalState[T] in mySqrt, the monadic sqrtH can remain unchanged :)

This is very confusing material, with no apparent conceptual commonality (ad hoc comes to mind) among the many examples I've looked at. Sometimes lift is used, other times a lift helper function, and in this case no use of lift at all.

That's because only methods of the MonadState and the MonadReader class are used and instances of MonadState are propagated/lifted through ReaderT, instance of MonadReader are propagated/lifted through StateT. ( instance MonadReader r m => MonadReader r (StateT s m) where     ask = lift ask     local = ... instance MonadState s m => MonadState (ReaderT r m) where     get = lift get     put = ... ) If you use a function on the inner monad which is not propagated to the entire transformer stack via class instances, you have to use lift (if you have a MonadTrans instance) or something similar.