Re: Give MonadTrans a QuantifiedConstraints superclass

+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. Adding `(>>==) :: Monad m => t m a -> (a -> t m b) -> t m b` seems even worse; it will break every single instance of MonadTrans in the ecosystem! 2021年6月3日(木) 9:45 Zemyla :

return is lift . return. Upon reflection, I believe (though may be mistaken) that every MonadTrans is also an instance of a hypothetical FunctorTrans class, only requiring a Functor constraint to be a Functor itself; however, ApplicativeTrans doesn't work because StateT among others requires a Monad constraint to be an Applicative.

On Wed, Jun 2, 2021 at 6:41 PM Carter Schonwald wrote:

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That sounds like a nice idea. Which laws would we require for it? The

usual monad laws require a pure too right? Along with fmap?

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Does this necessitate the existence of applicative trans?

On Wed, Jun 2, 2021 at 12:06 PM Zemyla wrote:

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I feel like instead, MonadTrans should have a function

(>>==) :: Monad m => t m a -> (a -> t m b) -> t m b

That way, it can prove it's a Monad while still staying Haskell 98.

On Wed, Jun 2, 2021, 10:51 Viktor Dukhovni

wrote:

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On Wed, Jun 02, 2021 at 07:27:28AM +0200, Henning Thielemann wrote:

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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. _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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