MonadFail instance for Either

[Daniel Bergey]

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 <david.feuer at gmail.com> 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 <ekmett at gmail.com> 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 <david.feuer at gmail.com>
>> 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 <bergey at alum.mit.edu>
>>> 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
>>> > _______________________________________________
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>>
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