On Sun, Dec 04, 2022 at 06:05:28PM +0100, Nicolas Trangez wrote:
For some project, I'm considering (again) to use an indexed state monad, which are now somewhat ergonomic to use with QualifiedDo.
When looking into existing packages exposing relevant types/classes, I had some questions:
1. These packages provide something like
``` class IxFunctor f where imap :: (a -> b) -> f i j a -> f i j b
class IxFunctor f => IxApplicative f where ... ```
Is this `IxFunctor` actually required/useful? It seems to me the implementation of `imap` for some `IxFunctor f` would be equal to `fmap` in the `Functor` implementation for `forall i j. f i j`. Hence, can `IxFunctor` be dropped in lieu of
``` class (forall i j. Functor (f i j)) => IxApplicative f where ... ```
I suspect QualifiedConstraints didn't exist at the time that class was introduced. I agree in modern GHC-flavoured Haskell the quantified constraint version is better. Tom