[Haskell-cafe] Hopefully simple monad question

Miguel Mitrofanov miguelimo38 at yandex.ru
Wed Sep 16 06:42:21 EDT 2009

Well, it's almost always better to reuse as much code as possible. But I 
don't think "type" is an answer here. I recommend using a "newtype", 
enabling "GeneralizedNewtypeDeriving" and deriving as much as possible:

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
newtype VM a =
   VM {runVM :: State VMState a} deriving Monad


newtype VMT m a =
   VMT {runVMT :: StateT VMState m a}
     deriving (Monad, MonadIO, MonadTrans, TransM)

Unfortunately, you can't automatically derive MonadState, since it's a 
multi-paremeter type class. You'll have to write down the derivation 

Gregory Propf wrote:
> I'm playing around with a little program that implements a simple 
> virtual machine.  I want to use a monad to represent machine state.  I 
> created a data type for the machine (VM) and a monadic type for the 
> monadic computations using it.  I declared this an instance of 
> MonadState and Monad and created the usual operators.  That stuff 
> works.  My issue is that I want to run some functions in the machine 
> monad, call it VMS - "virtual machine w/state" and then pull the 
> underlying VM data structure out and print it.
> I've read about monad transformers, lift, liftM, liftIO and all these 
> instances in the libraries like MonadIO and am rather confused.  The 
> most sensible conclusion I can reach is that I probably need to create 
> my own Transformer monad and define liftIO.  Is this where I need to 
> go?  Also, my VMS monad doesn't really do anything different from the 
> State monad except explicitly specify that state is a VM and not a 
> generic type.  Am I doing too much work creating my own instances here?  
> Would a simple "type" statement work?
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