Proposal: Remove the bogus MonadFail instance for ST

[Simon Peyton Jones]

I like the sound of Michael’s definition.  Can we document it with the MonadFail class, so that people making instances can find it easily?

Simon

From: Libraries [mailto:libraries-bounces at haskell.org] On Behalf Of Michael Snoyman
Sent: 14 March 2018 14:31
To: Ryan Scott <ryan.gl.scott at gmail.com>
Cc: Haskell Libraries <libraries at haskell.org>
Subject: Re: Proposal: Remove the bogus MonadFail instance for ST

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<mailto: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<mailto: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<mailto: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<mailto: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<mailto: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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