Give MonadTrans a QuantifiedConstraints superclass

[George Wilson]

+1 to the quantified constraint. It would be nice to be able to talk
about monad transformer composition.

Cheers,
George

On Thu, 3 Jun 2021 at 19:08, Edward Kmett <ekmett at gmail.com> wrote:
>
>
>
> On Wed, Jun 2, 2021 at 11:07 PM Henning Thielemann <lemming at henning-thielemann.de> wrote:
>>
>>
>> On Thu, 3 Jun 2021, Fumiaki Kinoshita wrote:
>>
>> > +1 to the original proposal of using QuantifiedConstraints.
>> >
>> > There is no need to stick to the standard from 23 years ago, and having
>> > two different classes is only likely to bring confusion and extra work
>> > for library maintainers.
>>
>> But why do we have the separation between 'transformers' and 'mtl' then?
>
>
> We have it though a combination of intent and historical accident.
>
> Originally there was just `mtl`-1. Then `transformers` + (`monads-fd` | `monads-tf`) came along with a couple of also-ran packages like Galois' `monadLib`. The ecosystem splintered badly and nobody could work with anybody in a different splinter. To merge the `mtl` portion of the ecosystem with the monads-fd ecosystem, we renamed the latter to `mtl`-2, and the `monads-tf` corner of the ecosystem silently died because nobody was using it and it conflicted in the module names with things that people did use.
>
> `transformers` was then sort of retroactively justified for two reasons, one to provide a "simple" core, then-Haskell98ish package like you suggest.
>
> `transformers` has supported things like PolyKinds for a long time, leans on StandaloneDeriving, etc. so it isn't exactly the first chink in that first justification's armor.
>
> Another justification was to supply a common base for experimentation on things like `monads-fd` and `monads-tf` and all the gajillion effect-system-alikes that have been built on top that do things like replace the `MonadFoo` classes with ones that use type families or tags or remove the functional dependencies.
>
> The latter mission remains intact.
>
> -Edward
>
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