Re: MonadFail instance for Either

I meant something like foo :: MonadFail m => A -> m Foo Sorry if my first version without input caused confusion. On October 25, 2018 11:48:44 AM UTC, Daniel Bergey wrote:

When a library provides a funct 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:

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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:

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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:

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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:

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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:

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Trac: https://ghc.haskell.org/trac/ghc/ticket/12160 libaries mailing list:

https://mail.haskell.org/pipermail/libraries/2016-August/027248.html

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Does anyone know of a later decision not to add it, or was it

simply no one's top priority?

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What is the next step to move this proposal forward? Is more

discussion in order? Should I just submit a patch?

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

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