On Sun, 2022-12-04 at 21:08 +0000, Tom Ellis wrote:
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.
Thanks, that confirms my intuition. Indeed, the aforementioned approach requires some language extensions not available when the existing packages were published. Cheers, Nicolas