Proposal: Remove the bogus MonadFail instance for ST

[Michael Snoyman]

I'd favor that decision as well, and could easily tweak my definition of
well behaved to simply be "no runtime exceptions or bottom values." My only
recommendation would be to start that as a separate proposal, as there's a
chance of more opposition to removing the IO instance than the ST instance.
I'd imagine IO will result in more breakage.

On Wed, Mar 14, 2018 at 4:34 PM, David Feuer <david.feuer at gmail.com> wrote:

> That seems reasonable. But I wonder if pattern matching failure in IO do
> should be allowed to slip by silently, or whether we should exclude the
> otherwise-reasonable instance to catch more mistakes.
>
> On Mar 14, 2018 10:31 AM, "Michael Snoyman" <michael at snoyman.com> wrote:
>
>> One possible "well behaved" intuition could be "cannot result in an
>> exception thrown from pure code without usage of unsafe functions." By this
>> definition:
>>
>> * Maybe's fail is well behaved: using `fail "foo"` results in a total
>> Nothing value
>> * List's: same thing, but with an empty list
>> * IO: runtime exception, but the exception is _not_ in pure code, but
>> rather from within IO, where exceptions are always to be expected
>> * ST: `runST (fail "foo")` results in a pure value which, when evaluated,
>> throws a runtime exception, breaking the well behaved definition
>> * Identity: `Identity (fail "foo")` can only be a pure value which throws
>> an exception, and is therefore not well behaved
>>
>> Note that I added the requirement of "without usage of unsafe functions,"
>> since `unsafePerformIO (fail "foo")` can result in a pure bottom value.
>>
>> On Wed, Mar 14, 2018 at 4:25 PM, Ryan Scott <ryan.gl.scott at gmail.com>
>> wrote:
>>
>>> 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/cb6d8589c83247ec96d5faa8
>>> 2df3e93f419bbfe0:/libraries/base/Control/Monad/Fail.hs#l80
>>> >>>>
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>>
>>
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