MonadFail instance for Either

[Daniel Bergey]

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 <bergey at alum.mit.edu> 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 <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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>>>