Avoiding lift with monad transformers

Question

I have a problem to which a stack of monad transformers (or even one monad transformer) over IO. Everything is good, except that using lift before every action is terribly annoying! I suspect there is really nothing to do about that, but I thought I'd ask anyway.

I am aware of lifting entire blocks, but what if the code is really of mixed types? Would it not be nice if GHC threw in some syntactic sugar (for example, <-$ = <- lift)?

Answer 1

For all the standard mtl monads, you don't need lift at all. get, put, ask, tell — they all work in any monad with the right transformer somewhere in the stack. The missing piece is IO, and even there liftIO lifts an arbitrary IO action down an arbitrary number of layers.

This is done with typeclasses for each "effect" on offer: for example, MonadState provides get and put. If you want to create your own newtype wrapper around a transformer stack, you can do deriving (..., MonadState MyState, ...) with the GeneralizedNewtypeDeriving extension, or roll your own instance:

instance MonadState MyState MyMonad where
  get = MyMonad get
  put s = MyMonad (put s)

You can use this to selectively expose or hide components of your combined transformer, by defining some instances and not others.

(You can easily extend this approach to all-new monadic effects you define yourself, by defining your own typeclass and providing boilerplate instances for the standard transformers, but all-new monads are rare; most of the time, you'll get by simply composing the standard set offered by mtl.)

Answer 2

You can make your functions monad-agnostic by using typeclasses instead of concrete monad stacks.

Let's say that you have this function, for example:

bangMe :: State String ()
bangMe = do
  str <- get
  put $ str ++ "!"
  -- or just modify (++"!")

Of course, you realize that it works as a transformer as well, so one could write:

bangMe :: Monad m => StateT String m ()

However, if you have a function that uses a different stack, let's say ReaderT [String] (StateT String IO) () or whatever, you'll have to use the dreaded lift function! So how is that avoided?

The trick is to make the function signature even more generic, so that it says that the State monad can appear anywhere in the monad stack. This is done like this:

bangMe :: MonadState String m => m ()

This forces m to be a monad that supports state (virtually) anywhere in the monad stack, and the function will thus work without lifting for any such stack.

There's one problem, though; since IO isn't part of the mtl, it doesn't have a transformer (e.g. IOT) nor a handy type class per default. So what should you do when you want to lift IO actions arbitrarily?

To the rescue comes MonadIO! It behaves almost identically to MonadState, MonadReader etc, the only difference being that it has a slightly different lifting mechanism. It works like this: you can take any IO action, and use liftIO to turn it into a monad agnostic version. So:

action :: IO ()
liftIO action :: MonadIO m => m ()

By transforming all of the monadic actions you wish to use in this way, you can intertwine monads as much as you want without any tedious lifting.