Re: [ghc-steering-committee] Proposal #631: Set program exit code by main return type, recommendation: accept something

20 Mar 2024

      On 2024-03-19 15:15, Arnaud Spiwack wrote:
...
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
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.
disable the warning.
...
From my understanding that is basically equivalent to what I was proposing.
It has the properties:
* Using a return type which could reasonably be expected to exit with failure
  should warn on -Wdefault (no matter whether a potential extension is enabled).
  * Concretely this means any return type but (), Void and maybe forall a. a, when the extension is off.
    * The warning could point to existing instances and the extension.
  * For anything which does not have an ExitCode implementation, when the extension is on.
* Using the ExitCode type class should be possible but gated behind an extension.

Now the question whether we should have two different warnings, one when the
extension is on and one when it is off or one warning which changes its wording
and behavior slightly depending on the extension being enabled does not matter
that much in my opinion.

Best
Malte
...
On Fri, 15 Mar 2024 at 15:32, Matthías Páll Gissurarson  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  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@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  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
...
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
...
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  wrote:
> I've added a comment to the GitHub thread
> (
>
https://github.com/ghc-proposals/ghc-proposals/pull/631#issuecomment-1983060...
)
>
> 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...
> <
>
https://github.com/shlevy/ghc-proposals/blob/io-exitcode/proposals/0631-main...
> .
> >
> > PS: I tend to agree with Richard that requiring an ExitStatus
instance
> > is the preferable option. But food for thought for the proposal
...
> > 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
> > 
> 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
> >     
> 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
> >          >         mailto:simon.peytonjones@gmail.com> wrote:
> >
> >                 I am a little worried about breaking programs
...
> >                 in an innocent-looking `return 0`, just because
some
> >                 other languages like to end programs with that
...
> >
> >
> >             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
> >              >             mailto:reisenberg@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
...
> >                 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
...
> >                 that end in an innocent-looking `return 0`, just
because
> >                 some other languages like to end programs with
...
> >                 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
> >                  >                 mailto:arnaud.spiwack@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
...
> >                     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 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
...
> >                     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
...
> >                     return value.
> >
> >                     The only change to the current behaviour is
...
> >                     `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
On 2024-03-14 14:32, Arnaud Spiwack wrote:
proposing)
thread
that end
phrase
programs
programs
that
the
the code.
there's
the
that
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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>
--
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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