While I’m weakly against the IsString version, I’m pretty strongly against both of these variants. They actively get in the way of a user who wants to treat the parameter uniformly.

-Edward

On Oct 25, 2018, at 8:06 AM, Daniel Bergey <bergey@teallabs.org> wrote:

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@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@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@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@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@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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