Deriving Via Bypass Transformer

BebeSparkelSparkel · April 17, 2024, 4:52pm

I am creating a monad transformer that I want to derive the MonadReader class for but ReaderT is part of the type. Is there a transformer that I could use with deriving via to skip over the ReaderT instance of MonadReader?

newtype FileT w m a = FileT (ReaderT Handle m a)
deriving via Bypass r m instance MonadReader r m => MonadReader r (FileT w m)

github.com/haskell/mtl

Bypass Coercible Type for Deriving Monad* Classes of New Transformers Composed of Transformers

haskell:master ← BebeSparkelSparkel:bypass

opened 08:08PM - 17 Apr 24 UTC

BebeSparkelSparkel

+41 -0

When creating a new transformer it is often composed of other transformers. The …transformers that it is composed of usually have specific purposes that are unrelated to the more general uses of the Monad* classes and typically you still want to define the Monad* instances for the underlying monad. My real world use case is the ```haskell newtype FileT w m a = FileT (ReaderT Handle m a) ``` where I still want to have MonadReader but not for reading Handle but rather the r for m `instance MonadReader r m => MonadReader r (FileT w m)` This is a bit annoying for this case but even more so for transformers wrapping RWS. In just the ReaderT case the following must be defined ```haskell instance MonadReader r m => MonadReader r (FileT w m) where local f (FileT (ReaderT g)) = FileT . ReaderT $ local f . g reader = lift . reader ``` DerivingVia seems to be a better option for this by using `Bypass` ```haskell deriving via ByPass (ReaderT Handle) m instance MonadReader r m => MonadReader r (FileT w m) ``` There could probably a more succinct way to write the derivation but this is what I have so far.

tomjaguarpaw · April 17, 2024, 5:04pm

Perhaps this works?

newtype Bypass t m a = Bypass (t m a)

instance MonadReader r m => MonadReader r (Bypass (ReaderT r') m) where
   ...

BebeSparkelSparkel · April 17, 2024, 5:18pm

Seems like that would work for MonadState and MonadWriter as well

tomjaguarpaw · April 17, 2024, 5:39pm

Yes, I think so.

BebeSparkelSparkel · April 17, 2024, 8:08pm

github.com/haskell/mtl

Bypass Coercible Type for Deriving Monad* Classes of New Transformers Composed of Transformers

haskell:master ← BebeSparkelSparkel:bypass

opened 08:08PM - 17 Apr 24 UTC

BebeSparkelSparkel

+41 -0

When creating a new transformer it is often composed of other transformers. The …transformers that it is composed of usually have specific purposes that are unrelated to the more general uses of the Monad* classes and typically you still want to define the Monad* instances for the underlying monad. My real world use case is the ```haskell newtype FileT w m a = FileT (ReaderT Handle m a) ``` where I still want to have MonadReader but not for reading Handle but rather the r for m `instance MonadReader r m => MonadReader r (FileT w m)` This is a bit annoying for this case but even more so for transformers wrapping RWS. In just the ReaderT case the following must be defined ```haskell instance MonadReader r m => MonadReader r (FileT w m) where local f (FileT (ReaderT g)) = FileT . ReaderT $ local f . g reader = lift . reader ``` DerivingVia seems to be a better option for this by using `Bypass` ```haskell deriving via ByPass (ReaderT Handle) m instance MonadReader r m => MonadReader r (FileT w m) ``` There could probably a more succinct way to write the derivation but this is what I have so far.

Vlix · April 17, 2024, 9:58pm

Since it’s a newtype around ReaderT, wouldn’t the following just work?
(Not sure if you’d need to add the Handle part)

newtype FileT ...
    deriving newtype MonadReader (Handle)

tomjaguarpaw · April 18, 2024, 11:12am

They want the MonadReader instance to come from the inner m, not the outer ReaderT.