Proposal: DerivingVia, MTL

[Reed Mullanix]

Sorry, I should've been more clear. The proposal is to add the Inner
newtype and it's associated instances to MTL.

On Sun, May 19, 2019, 7:57 PM chessai ., <chessai1996 at gmail.com> wrote:

> 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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>> Libraries at haskell.org
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>>
>
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