Proposal: DerivingVia, MTL

[chessai .]

I'm confused. This seems more like a helpful tip than a proposal. Where is
the proposal?

On Sun, May 19, 2019, 9:27 PM Reed Mullanix <reedmullanix at gmail.com> wrote:

> 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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