Re: MonadFail instance for Either

instance MonadFail (Either [Char]) would satisfy all of the cases where I want this instance. So would the suggestion on Trac of instance e ~ String => MonadFail (Either e) although that probably runs afoul of David's concerns about standards. How do other people consume MonadFail? Maybe I'm missing an existing solution. When a library provides a function foo :: MonadFail m => m Foo I usually want to recover from failure while logging the failure message. I can do this with the IO instance, but then it's not obvius that all errors are getting caught. If I'm not in IO, and I want the error text, I think I'm out of luck. On October 25, 2018 7:21:51 AM UTC, David Feuer wrote:

Another option is to be agnostic about it with FlexibleInstances:

instance MonadFail (Either [Char]) where fail = Left

That'll work today, and leave the question of the ultimate constraint open. It's not Haskell 2010, but no one can take advantage of that fact.

I'm only raising that as an option; I don't really like it terribly much.

On Thu, Oct 25, 2018, 3:05 AM Edward Kmett wrote:

...

I'm also weakly inclined against it.

If we decided we really wanted something, then building a class that was just for this purpose might work, sort of an updated version of the old 'Error' class from transformers, but now limited to just the failure string so it has no extra baggage.

On the other hand, that then faces inertia problems all its own.

-Edward

On Wed, Oct 24, 2018 at 11:42 PM David Feuer wrote:

...

FWIW, I think I'm weakly opposed. Either is Haskell 98. MonadFail is solidly "standards-track" material, to the extent that designation is meaningful at the moment. IsString ... isn't. On Wed, Oct 24, 2018 at 10:44 PM Daniel Bergey wrote:

...

Is there still consensus in favor of adding this instance?

instance IsString str => MonadFail (Either str) where fail = Left . fromString

In 2016 there was some discussion, and my reading is that there

was consensus in favor at the time:

...

Trac: https://ghc.haskell.org/trac/ghc/ticket/12160 libaries mailing list: https://mail.haskell.org/pipermail/libraries/2016-August/027248.html

Does anyone know of a later decision not to add it, or was it simply no one's top priority?

What is the next step to move this proposal forward? Is more discussion in order? Should I just submit a patch?

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

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