Give MonadTrans a QuantifiedConstraints superclass

[Edward Kmett]

On Wed, Jun 2, 2021 at 9:06 AM Zemyla <zemyla at gmail.com> wrote:

> I feel like instead, MonadTrans should have a function
>
> (>>==) :: Monad m => t m a -> (a -> t m b) -> t m b
>

This strikes me as a strictly worse outcome. Now you get coherence laws
relating (>>==) to a (>>=) that may or may not exist that you have to keep
track of, but get nothing enforcing anything, can't delegate to code that
builds off Monad, leading to random code duplication, and users are hoist
on the horns of the dilemma of using (>>=) or (>>==) with
different constraints in each circumstance.

-Edward


> That way, it can prove it's a Monad while still staying Haskell 98.
>
> On Wed, Jun 2, 2021, 10:51 Viktor Dukhovni <ietf-dane at dukhovni.org> wrote:
>
>> On Wed, Jun 02, 2021 at 07:27:28AM +0200, Henning Thielemann wrote:
>>
>> > 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.
>>
>> I don't think that having two incompatible MonadTrans classes would
>> constitute progress.  Older versions of the transformers library (which
>> is by now quite stable) will continue to be available, for anyone who
>> wants to use a Haskell '98 (ish?) version.
>>
>> [ FWIW, I don't know what you mean by "is mostly Haskell '98", I'd
>>   expect that to be a strict binary choice: is or isn't. ]
>>
>> --
>>     Viktor.
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