Proposal: DerivingVia, MTL

[Reed Mullanix]

As it stands, working with newtypes around transformers can be kind of a
pain. For example,
consider

newtype FreshT m a = { unFreshT :: StateT Int m a }
  deriving (Functor, Applicative, Monad, MonadReader r, ...)

Right now, we can't just GND the instance:

instance (MonadState s m) => MonadState s (FreshT m)

However, the instance we end up writing is completely formulaic.
Fortunately, we can solve this by using the recently added -XDerivingVia

newtype Inner (t :: (* -> *) -> * -> *) m a = Inner { getInner :: t m a }
  deriving (Functor, Applicative, Monad)

instance (MonadState s m) => MonadState s (Inner (StateT s') m) where
  get = Inner $ lift get
  put = Inner . lift . put

This lets us derive the instance that we were looking for

newtype FreshT m a = FreshT { unFreshT :: StateT Int m a }
  deriving newtype (Functor, Applicative, Monad, MonadReader r)
  deriving (MonadState s) via Inner (StateT Int) m

This can be extended to other transformers/classes very easily. It also
works well when dealing with newtyped transformer stacks.

newtype FooT s e m a = FooT { unFooT :: StateT s (WriterT String (ExceptT e
m)) a }
  deriving newtype (Functor, Applicative, Monad, MonadError e)
  deriving (MonadWriter w) via (StateT s (Inner (WriterT String) (ExceptT e
m)))
  deriving (MonadState s') via Inner (StateT s) (WriterT String (ExceptT e
m))

Cheers,
Reed Mullanix
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