Why can't arguments be levity polymorphic for inline functions?

[Clinton Mead]

Ben,

The suggestion of erroring if the inline pragma was not there was just
because I thought it would be better than silently doing something
different. But that's just a subjective opinion, it's not core to what I'm
proposing.

Indeed there are two other options:

1. Make levity polymorphic functions implicitly inline OR
2. Compile a version which wraps all the levity polymorphism in boxes.

Either approach would mean the program would still be accepted with or
without the pragma. Whether either of them are a good idea is debatable,
but it shows it's not necessary to require a pragma.

So if it's important that excluding a pragma doesn't result in a program
being rejected, either of the above options would solve that issue.

On Sat, Oct 9, 2021 at 2:06 AM Ben Gamari <ben at smart-cactus.org> wrote:

> Chris Smith <cdsmith at gmail.com> writes:
>
> > On Fri, Oct 8, 2021 at 10:51 AM Ben Gamari <ben at smart-cactus.org> wrote:
> >
> >> In my mind the fundamental problem with this approach is that it means
> >> that a program's acceptance by the compiler hinges upon pragmas.
> >> This is a rather significant departure from the status quo, where one
> >> can remove all pragmas and still end up with a well-formed program.
> >> In this sense, pragmas aren't really part of the Haskell language but
> >> are rather bits of interesting metadata that the compiler may or may not
> >> pay heed to.
> >>
> >
> > I don't believe this is really the status quo.  In particular, the
> pragmas
> > relating to overlapping instances definitely do affect whether a program
> > type-checks or not.
>
> Yes, this is a fair point. Moreover, the same can be said of
> LANGUAGE pragmas more generally. I will rephrase my statement to reflect
> what was in my head when I initially wrote it:
>
> >> In my mind the fundamental problem with this approach is that it means
> >> that a program's acceptance by the compiler hinges upon INLINE pragmas.
> >> This is a rather significant departure from the status quo, where one
> >> can remove all INLINE, INLINEABLE, RULES, and SPECIALISE pragmas and
> >> still end up with a well-formed program.
>
> These pragmas all share the property that they don't change program
> semantics but rather merely affect operational behavior. Consequently,
> they should not change whether a program should be accepted.
>
> Cheers,
>
> - Ben
>
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