[ghc-steering-committee] Proposal #631: Set program exit code by main return type, recommendation: accept something
Arnaud Spiwack
arnaud.spiwack at tweag.io
Tue Mar 19 14:15:25 UTC 2024
So, my opinion that I promised, is that the best would probably be to have
add a warning when main doesn't have type `IO ()` or `IO Void` without the
extension (the behaviour is otherwise unchanged), and then change the
behaviour to use the typeclass with an extension (which we hope to turn on
by default in the next language edition). The extension should probably
disable the warning.
Now, I don't actually know how to implement this properly. So maybe it's a
bad idea. But I believe that's what I'd like to see.
On Fri, 15 Mar 2024 at 15:32, Matthías Páll Gissurarson <mpg at mpg.is> wrote:
> I agree. I like it, but better behind an extension to avoid surprises and
> enable experimentation.
>
> On Fri, 15 Mar 2024 at 13:15, Simon Marlow <marlowsd at gmail.com> wrote:
>
>> I also think any change in behaviour should be behind an extension.
>>
>> Cheers
>> Simon
>>
>> On Thu, 14 Mar 2024 at 16:51, Simon Peyton Jones <
>> simon.peytonjones at gmail.com> wrote:
>>
>>> I like the proposal basically as is. i.e. typeclass + warning
>>> Especially the fact that it warns everyone and breaks no-one (who
>>> doesn’t want
>>> to).
>>>
>>> I am weakly in favor of gating the usage of the typeclass for anything
>>> but ()
>>> and Void behind an extension. That way no program will newly exit with
>>> failure
>>> without the user opting in and we might be able to experiment more on
>>> the type
>>> class.
>>>
>>>
>>> I'm with Malte. But I don't have strongly held views.
>>>
>>> Simon
>>>
>>> On Thu, 14 Mar 2024 at 14:04, Malte Ott <malte.ott at maralorn.de> wrote:
>>>
>>>> I like the proposal basically as is. i.e. typeclass + warning
>>>> Especially the fact that it warns everyone and breaks no-one (who
>>>> doesn’t want
>>>> to).
>>>>
>>>> I am weakly in favor of gating the usage of the typeclass for anything
>>>> but ()
>>>> and Void behind an extension. That way no program will newly exit with
>>>> failure
>>>> without the user opting in and we might be able to experiment more on
>>>> the type
>>>> class.
>>>>
>>>> Best
>>>> Malte
>>>>
>>>> On 2024-03-14 14:32, Arnaud Spiwack wrote:
>>>> > Dear all,
>>>> >
>>>> > Shea has updated his proposal based on the committee's feedback.
>>>> >
>>>> > There seem to be two main alternatives being considered at the moment
>>>> > - Having a type class to compute the exit code based on the type.
>>>> This is
>>>> > Shea's favourite. It can be done without an extension (as Shea's
>>>> proposing)
>>>> > or with an extension.
>>>> > - Keep the current behaviour but emit a warning when the return type
>>>> of
>>>> > `main` isn't `()` or `Void`.
>>>> >
>>>> > I have opinions about my preference, but I'd like to hear about
>>>> everybody's
>>>> > thoughts first.
>>>> >
>>>> > On Thu, 7 Mar 2024 at 10:27, Adam Gundry <adam at well-typed.com> wrote:
>>>> >
>>>> > > I've added a comment to the GitHub thread
>>>> > > (
>>>> > >
>>>> https://github.com/ghc-proposals/ghc-proposals/pull/631#issuecomment-1983060484
>>>> )
>>>> > >
>>>> > > elaborating slightly on Richard's suggestion (albeit with an
>>>> effectively
>>>> > > indefinite transition period).
>>>> > >
>>>> > > Adam
>>>> > >
>>>> > >
>>>> > > On 05/03/2024 08:52, Arnaud Spiwack wrote:
>>>> > > > This is Alternative 7.5 in the current version of the proposal
>>>> > > >
>>>> > >
>>>> https://github.com/shlevy/ghc-proposals/blob/io-exitcode/proposals/0631-main-return-types.rst#75require-an-exitstatus-instance
>>>> > > <
>>>> > >
>>>> https://github.com/shlevy/ghc-proposals/blob/io-exitcode/proposals/0631-main-return-types.rst#75require-an-exitstatus-instance
>>>> >
>>>> > > .
>>>> > > >
>>>> > > > PS: I tend to agree with Richard that requiring an ExitStatus
>>>> instance
>>>> > > > is the preferable option. But food for thought for the proposal
>>>> thread
>>>> > > > when that conversation happens there: should that be gated behind
>>>> an
>>>> > > > extension? In which case it won't become the default before the
>>>> next
>>>> > > > language edition.
>>>> > > >
>>>> > > > /Arnaud
>>>> > > >
>>>> > > > On Mon, 4 Mar 2024 at 21:35, Simon Peyton Jones
>>>> > > > <simon.peytonjones at gmail.com <mailto:simon.peytonjones at gmail.com
>>>> >>
>>>> > > wrote:
>>>> > > >
>>>> > > > I left out a key part of my last email -- apologies. I'm
>>>> > > > floating a counter-proposal where we *require* an
>>>> instance of
>>>> > > > ExitStatus on the return type of `main`, with a transition
>>>> > > > period. In contrast, my understanding of the proposal
>>>> written is
>>>> > > > that it would use such an instance if it exists, but
>>>> issue a
>>>> > > > warning if it doesn't, in perpetuity.
>>>> > > >
>>>> > > >
>>>> > > > Ah I had not realised that.
>>>> > > >
>>>> > > > But why?
>>>> > > >
>>>> > > > Rather than answer here (private to SC) why don't you put your
>>>> > > > proposal on the discussion thread, say why, and invite
>>>> feedback.
>>>> > > >
>>>> > > > Simon
>>>> > > >
>>>> > > >
>>>> > > > On Mon, 4 Mar 2024 at 19:24, Richard Eisenberg
>>>> > > > <reisenberg at janestreet.com <mailto:reisenberg at janestreet.com
>>>> >>
>>>> > > wrote:
>>>> > > >
>>>> > > > I left out a key part of my last email -- apologies. I'm
>>>> > > > floating a counter-proposal where we *require* an
>>>> instance of
>>>> > > > ExitStatus on the return type of `main`, with a transition
>>>> > > > period. In contrast, my understanding of the proposal
>>>> written is
>>>> > > > that it would use such an instance if it exists, but
>>>> issue a
>>>> > > > warning if it doesn't, in perpetuity.
>>>> > > >
>>>> > > > Richard
>>>> > > >
>>>> > > > On Mon, Mar 4, 2024 at 6:14 AM Simon Peyton Jones
>>>> > > > <simon.peytonjones at gmail.com
>>>> > > > <mailto:simon.peytonjones at gmail.com>> wrote:
>>>> > > >
>>>> > > > I am a little worried about breaking programs
>>>> that end
>>>> > > > in an innocent-looking `return 0`, just because
>>>> some
>>>> > > > other languages like to end programs with that
>>>> phrase
>>>> > > >
>>>> > > >
>>>> > > > The proposal specifies that such a program returns
>>>> > > > `ExitSuccess`, but adds a warning. That seems OK to
>>>> me; it
>>>> > > > does not break the program.
>>>> > > >
>>>> > > > Oh -- maybe you mean that `return 1` means "return
>>>> with exit
>>>> > > > code 1" today. Is that really true? I don't think
>>>> so.
>>>> > > >
>>>> > > > Overall this proposal seems fine to me. I'd be happy
>>>> to see
>>>> > > > it done.
>>>> > > >
>>>> > > > Simon
>>>> > > >
>>>> > > > On Thu, 29 Feb 2024 at 12:38, Richard Eisenberg
>>>> > > > <reisenberg at janestreet.com
>>>> > > > <mailto:reisenberg at janestreet.com>> wrote:
>>>> > > >
>>>> > > > I haven't followed this proposal closely. But
>>>> couldn't
>>>> > > > we have a transition period toward this eventual
>>>> goal?
>>>> > > > That is, introduce a new warning, on by default,
>>>> if
>>>> > > > `main` returns anything other than `()`. That
>>>> goes for a
>>>> > > > few releases. Then we require that the return
>>>> type of
>>>> > > > main has an instance of ExitStatus.
>>>> > > >
>>>> > > > I'm not worried about changing the behavior of
>>>> programs
>>>> > > > that have type IO ExitCode but expect the program
>>>> to
>>>> > > > return 0 unconditionally; that's just begging for
>>>> > > > confusion. I am a little worried about breaking
>>>> programs
>>>> > > > that end in an innocent-looking `return 0`, just
>>>> because
>>>> > > > some other languages like to end programs with
>>>> that
>>>> > > > phrase. So I'm not sure if we should have an
>>>> instance
>>>> > > > ExitStatus Int (or instance ExitStatus Integer)
>>>> -- but
>>>> > > > we probably should. If a program ends with
>>>> `return 1`,
>>>> > > > the programmer probably wants the OS to return 1
>>>> as well.
>>>> > > >
>>>> > > > Richard
>>>> > > >
>>>> > > > On Thu, Feb 29, 2024 at 5:29 AM Arnaud Spiwack
>>>> > > > <arnaud.spiwack at tweag.io
>>>> > > > <mailto:arnaud.spiwack at tweag.io>> wrote:
>>>> > > >
>>>> > > > Dear all,
>>>> > > >
>>>> > > > Shea Levy proposes to do something with the
>>>> values
>>>> > > > returned by `main`
>>>> > > >
>>>> > > https://github.com/ghc-proposals/ghc-proposals/pull/631 <
>>>> > > https://github.com/ghc-proposals/ghc-proposals/pull/631> .
>>>> > > >
>>>> > > > The problem is that `main` is allowed to be
>>>> of type
>>>> > > > `IO A` for any `A`. And GHC will simply drop
>>>> the
>>>> > > > value returned by `main`. Shea contends that
>>>> it's
>>>> > > > surprising. I agree that dropping a value
>>>> without
>>>> > > > the compiler being explicitly instructed to is
>>>> > > > surprising. But Shea says that when `A` is
>>>> > > > `ExitCode` this is even more surprising.
>>>> Namely
>>>> > > > `main :: IO ExitCode; main = return $ Failure
>>>> 1`
>>>> > > > actually terminates with exit code 0. And I
>>>> doubt
>>>> > > > that it's what anybody expects when reading
>>>> the code.
>>>> > > >
>>>> > > > The proposal is simple, but I have a lot of
>>>> comments
>>>> > > > on it. Sorry about that…
>>>> > > >
>>>> > > > Now, this sort of proposal is tricky. When the
>>>> > > > current behaviour is confusing, we want to
>>>> change
>>>> > > > the default. But putting the new default
>>>> behind an
>>>> > > > extension doesn't really solve the fact that
>>>> there's
>>>> > > > a trap. The extension is, therefore, unlikely
>>>> to be
>>>> > > > well tested before it becomes part of the next
>>>> > > > language edition.
>>>> > > >
>>>> > > > Shea's main proposition doesn't actually use
>>>> an
>>>> > > > extension though. He adds a type class
>>>> `ExitStatus`,
>>>> > > > and if `ExistStatus A`, then `main :: IO A`
>>>> uses the
>>>> > > > instance to determine the exit code based on
>>>> the
>>>> > > > return value.
>>>> > > >
>>>> > > > The only change to the current behaviour is
>>>> that
>>>> > > > `main :: IO ExitCode` instead of always
>>>> terminating
>>>> > > > with exit code 0 when returning now
>>>> terminates with
>>>> > > > the expected error code. The argument for not
>>>> > > > putting this behind an extension is that
>>>> virtually
>>>> > > > anybody affected by the change will actually
>>>> have
>>>> > > > the behaviour they were expecting. But maybe
>>>> the
>>>> > > > argument isn't strong enough (the changes may
>>>> be
>>>> > > > more “interesting” if some library exports
>>>> some
>>>> > > > `ExistStatus` instance).
>>>> > > >
>>>> > > > This design of this proposal is inspired by
>>>> Rust's
>>>> > > > design. I've asked our Rust team, and they
>>>> certainly
>>>> > > > seem to have internalised the idea of
>>>> returning an
>>>> > > > exit code. It really seems a pretty natural
>>>> feature
>>>> > > > to have. So I'm rather in favour of some
>>>> flavour of
>>>> > > > the type class implementation. Though have a
>>>> look at
>>>> > > > the alternatives, where you'll find other
>>>> approaches
>>>> > > > such as restricting the type of `main` to
>>>> > > > unsurprising types.
>>>> > > >
>>>> > > > One caveat with respect to the main proposal:
>>>> it is
>>>> > > > proposed that when no `ExistStatus A` is
>>>> found, then
>>>> > > > we drop the returned value like today. I
>>>> don't know
>>>> > > > that it's quite easy to implement this
>>>> behaviour.
>>>> > > > But it can be recovered by a catch-all
>>>> overlapping
>>>> > > > instance, so maybe it's a better way to
>>>> specify the
>>>> > > > desired behaviour.
>>>> > > >
>>>> > > > --
>>>> > > > Arnaud Spiwack
>>>> > > > Director, Research at https://moduscreate.com
>>>> > > > <https://moduscreate.com> and
>>>> https://tweag.io
>>>> > > > <https://tweag.io>.
>>>> > >
>>>> > >
>>>> > > --
>>>> > > Adam Gundry, Haskell Consultant
>>>> > > Well-Typed LLP, https://www.well-typed.com/
>>>> > >
>>>> > > Registered in England & Wales, OC335890
>>>> > > 27 Old Gloucester Street, London WC1N 3AX, England
>>>> > >
>>>> > > _______________________________________________
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>>>> > > ghc-steering-committee at haskell.org
>>>> > >
>>>> https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee
>>>> > >
>>>> >
>>>> >
>>>> > --
>>>> > Arnaud Spiwack
>>>> > Director, Research at https://moduscreate.com and https://tweag.io.
>>>>
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>
>
> --
> -- Matthías Páll Gissurarson <http://mpg.is/>
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--
Arnaud Spiwack
Director, Research at https://moduscreate.com and https://tweag.io.
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