Proposal: Remove the bogus MonadFail instance for ST

[Ryan Scott]

Thanks, that makes more sense. I'm inclined to agree that MonadFail
instances should fail in a "well-behaved" way. (I wish I knew how to
make the phrase "well-behaved" more formal, but I don't.) It might be
worth adding this intuition to the Haddocks for MonadFail.

That being said, one thing to consider before removing this instance
is that there will be some breakage. Ben Gamari added this instance in
[1] because apparently the regex-tdfa package needed it. Other than
that, though, I don't have any real objections to removing this
instance.

Ryan S.
-----
[1] https://phabricator.haskell.org/D3982

On Wed, Mar 14, 2018 at 9:58 AM, David Feuer <david.feuer at gmail.com> wrote:
> I expect a MonadFail instance to have a well-behaved notion of failure
> within the monad. An exception from "pure" code (which is what ST
> simulates) is not that. On the other hand, perhaps you're right and
> the instance should be removed for IO as well; I don't have as strong
> a sense of revulsion, but maybe users should be forced to be explicit
> with throwIO.
>
> On Wed, Mar 14, 2018 at 9:46 AM, Ryan Scott <ryan.gl.scott at gmail.com> wrote:
>> OK. You used the phrase "utterly contrary to the purpose of
>> MonadFail", so I'm trying to figure out exactly what you mean here.
>> Prima facie, the purpose of MonadFail (at least, as explained in its
>> Haddocks) is to provide a type class–directed way of desugaring
>> partial pattern matches in do-notation. With this in mind, the current
>> MonadFail instance for ST doesn't seem too offensive.
>>
>> However, I think you have some additional property in mind that you
>> feel the MonadFail ST instance runs afoul of. Do you mind explaining
>> in further detail what this is? (I'm not trying to be snarky here—I
>> genuinely don't know what you're getting at.)
>>
>> Ryan S.
>>
>> On Wed, Mar 14, 2018 at 9:41 AM, David Feuer <david.feuer at gmail.com> wrote:
>>> I am not. I think that instance is fairly legitimate, as it raises an
>>> IO exception that can be caught in IO. IO's Alternative instance is a
>>> bit shadier, but that's not a topic for this proposal either. ST is an
>>> entirely different story, and I'm sorry I accidentally mixed it in.
>>>
>>> On Wed, Mar 14, 2018 at 9:05 AM, Ryan Scott <ryan.gl.scott at gmail.com> wrote:
>>>> It's worth noting that the MonadFail instance for IO [1] also simply throws
>>>> an error (by way of failIO). Are you proposing we remove this instance as
>>>> well?
>>>>
>>>> Ryan S.
>>>> -----
>>>> [1]
>>>> http://git.haskell.org/ghc.git/blob/cb6d8589c83247ec96d5faa82df3e93f419bbfe0:/libraries/base/Control/Monad/Fail.hs#l80
>>>>
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