On Wed, 2 Jun 2021, Baldur Blöndal wrote:
This is to advertise the proposal (https://gitlab.haskell.org/ghc/ghc/-/issues/19922) to add a superclass to the MonadTrans type class in Control.Monad.Trans.
A Monad transformer 'trans' lifts a 'Monad m' to a 'Monad (trans m)'.
This proposal code-ifies that with a superclass constraint:
class (forall m. Monad m => Monad (trans m)) => MonadTrans trans where ..
This is the main motiviating example of the Quantified Class Constraints paper https://gkaracha.github.io/papers/quantcs.pdf
So far, 'transformers' is mostly Haskell 98. This is why I prefer it to 'mtl'. Wouldn't it be enough to add this extension to 'mtl'? I see that 'mtl' re-uses the MonadTrans class from 'transformers' but maybe it should define its own class with the quantified constraints then.