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
or
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
yourself.
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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