[ghc-steering-committee] Proposal #631: Set program exit code by main return type, recommendation: accept something
Simon Peyton Jones
simon.peytonjones at gmail.com
Thu Mar 14 16:51:03 UTC 2024
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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